feat: add comprehensive Rust implementation with feature parity

This commit completes the Rust implementation of mail2couch with full feature
parity to the Go version, including:

- Complete IMAP client with TLS support and retry logic
- Advanced email parsing with MIME multipart support using mail-parser
- Email attachment extraction and CouchDB storage
- Sync mode implementation with deleted message handling
- Enhanced error handling and retry mechanisms
- Identical command-line interface with bash completion
- Test configurations for both implementations

The Rust implementation now provides:
- Memory safety and type safety guarantees
- Modern async/await patterns with tokio/async-std
- Comprehensive error handling with anyhow/thiserror
- Structured logging and progress reporting
- Performance optimizations and retry logic

Test configurations created:
- rust/config-test-rust.json - Rust implementation test config
- go/config-test-go.json - Go implementation test config
- test-config-comparison.md - Detailed comparison documentation
- test-both-implementations.sh - Automated testing script

Both implementations can now be tested side-by-side with identical
configurations to validate feature parity and performance.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

rust/src/couch.rs

@@ -8,6 +8,7 @@ use crate::schemas::{MailDocument, SyncMetadata};
 use anyhow::{anyhow, Result};
 use reqwest::{Client, StatusCode};
 use serde_json::Value;
+use std::time::Duration;
 use thiserror::Error;
 
 #[derive(Error, Debug)]
@@ -42,6 +43,62 @@ pub struct CouchResponse {
 }
 
 impl CouchClient {
+    /// Generic retry helper for CouchDB operations
+    async fn retry_operation<F, Fut, T>(&self, operation_name: &str, operation: F) -> Result<T>
+    where
+        F: Fn() -> Fut,
+        Fut: std::future::Future<Output = Result<T>>,
+    {
+        const MAX_RETRIES: u32 = 3;
+        const RETRY_DELAY_MS: u64 = 1000;
+
+        let mut last_error = None;
+
+        for attempt in 1..=MAX_RETRIES {
+            match operation().await {
+                Ok(result) => {
+                    if attempt > 1 {
+                        log::debug!("✅ CouchDB {} successful on attempt {}", operation_name, attempt);
+                    }
+                    return Ok(result);
+                }
+                Err(e) => {
+                    // Check if this is a retryable error
+                    let is_retryable = match &e.downcast_ref::<CouchError>() {
+                        Some(CouchError::Http(_)) => true,  // Network errors are retryable
+                        Some(CouchError::CouchDb { status, .. }) => {
+                            // Retry on server errors (5xx) but not client errors (4xx)
+                            *status >= 500
+                        }
+                        _ => false, // Other errors are not retryable
+                    };
+
+                    last_error = Some(e);
+                    
+                    if is_retryable && attempt < MAX_RETRIES {
+                        log::warn!(
+                            "🔄 CouchDB {} attempt {} failed, retrying in {}ms: {}",
+                            operation_name,
+                            attempt,
+                            RETRY_DELAY_MS,
+                            last_error.as_ref().unwrap()
+                        );
+                        tokio::time::sleep(Duration::from_millis(RETRY_DELAY_MS)).await;
+                    } else {
+                        break;
+                    }
+                }
+            }
+        }
+
+        Err(anyhow!(
+            "CouchDB {} failed after {} attempts. Last error: {}",
+            operation_name,
+            MAX_RETRIES,
+            last_error.unwrap()
+        ))
+    }
+
     /// Create a new CouchDB client
     pub fn new(config: &CouchDbConfig) -> Result<Self> {
         let client = Client::new();
@@ -115,22 +172,68 @@ impl CouchClient {
         Ok(response.status().is_success())
     }
 
-    /// Store a mail document in CouchDB
+    /// Store a mail document in CouchDB with optional attachments and retry logic
     pub async fn store_mail_document(&self, db_name: &str, mut document: MailDocument) -> Result<String> {
         // Set the document ID if not already set
         if document.id.is_none() {
             document.set_id();
         }
 
-        let doc_id = document.id.as_ref().unwrap();
+        let doc_id = document.id.as_ref().unwrap().clone();
         
         // Check if document already exists to avoid duplicates
-        if self.document_exists(db_name, doc_id).await? {
-            return Ok(doc_id.clone());
+        if self.document_exists(db_name, &doc_id).await? {
+            return Ok(doc_id);
         }
 
-        let url = format!("{}/{}/{}", self.base_url, db_name, doc_id);
-        let mut request = self.client.put(&url).json(&document);
+        self.retry_operation("store_mail_document", || async {
+            let url = format!("{}/{}/{}", self.base_url, db_name, doc_id);
+            let mut request = self.client.put(&url).json(&document);
+            
+            if let Some((username, password)) = &self.auth {
+                request = request.basic_auth(username, Some(password));
+            }
+
+            let response = request.send().await
+                .map_err(|e| CouchError::Http(e))?;
+            
+            match response.status() {
+                StatusCode::CREATED | StatusCode::ACCEPTED => {
+                    let couch_response: CouchResponse = response.json().await
+                        .map_err(|e| CouchError::Http(e))?;
+                    Ok(couch_response.id.unwrap_or_else(|| doc_id.clone()))
+                }
+                status => {
+                    let error_text = response.text().await
+                        .unwrap_or_else(|_| "Failed to read error response".to_string());
+                    Err(CouchError::CouchDb {
+                        status: status.as_u16(),
+                        message: error_text,
+                    }.into())
+                }
+            }
+        }).await
+    }
+
+    /// Store an attachment for a document in CouchDB
+    pub async fn store_attachment(
+        &self,
+        db_name: &str,
+        doc_id: &str,
+        attachment_name: &str,
+        content_type: &str,
+        data: &[u8],
+    ) -> Result<String> {
+        // First get the current document revision
+        let doc_response = self.get_document_rev(db_name, doc_id).await?;
+        let rev = doc_response.ok_or_else(|| anyhow!("Document {} not found", doc_id))?;
+
+        // Upload the attachment
+        let url = format!("{}/{}/{}/{}?rev={}", self.base_url, db_name, doc_id, attachment_name, rev);
+        let mut request = self.client
+            .put(&url)
+            .header("Content-Type", content_type)
+            .body(data.to_vec());
         
         if let Some((username, password)) = &self.auth {
             request = request.basic_auth(username, Some(password));
@@ -141,11 +244,35 @@ impl CouchClient {
         match response.status() {
             StatusCode::CREATED | StatusCode::ACCEPTED => {
                 let couch_response: CouchResponse = response.json().await?;
-                Ok(couch_response.id.unwrap_or_else(|| doc_id.clone()))
+                Ok(couch_response.rev.unwrap_or_else(|| rev))
             }
             status => {
                 let error_text = response.text().await?;
-                Err(anyhow!("Failed to store document {}: {} - {}", doc_id, status, error_text))
+                Err(anyhow!("Failed to store attachment {}: {} - {}", attachment_name, status, error_text))
+            }
+        }
+    }
+
+    /// Get document revision
+    async fn get_document_rev(&self, db_name: &str, doc_id: &str) -> Result<Option<String>> {
+        let url = format!("{}/{}/{}", self.base_url, db_name, doc_id);
+        let mut request = self.client.get(&url);
+        
+        if let Some((username, password)) = &self.auth {
+            request = request.basic_auth(username, Some(password));
+        }
+
+        let response = request.send().await?;
+        
+        match response.status() {
+            StatusCode::OK => {
+                let doc: Value = response.json().await?;
+                Ok(doc["_rev"].as_str().map(|s| s.to_string()))
+            }
+            StatusCode::NOT_FOUND => Ok(None),
+            status => {
+                let error_text = response.text().await?;
+                Err(anyhow!("Failed to get document {}: {} - {}", doc_id, status, error_text))
             }
         }
     }
@@ -244,6 +371,46 @@ impl CouchClient {
         }
     }
 
+    /// Get all message UIDs for a specific mailbox from CouchDB
+    pub async fn get_mailbox_uids(&self, db_name: &str, mailbox: &str) -> Result<Vec<u32>> {
+        let url = format!("{}/{}/_all_docs", self.base_url, db_name);
+        let query_params = [
+            ("startkey", format!("\"{}\"", mailbox)),
+            ("endkey", format!("\"{}\\ufff0\"", mailbox)), // High Unicode character for range end
+            ("include_docs", "false".to_string()),
+        ];
+
+        let mut request = self.client.get(&url).query(&query_params);
+        
+        if let Some((username, password)) = &self.auth {
+            request = request.basic_auth(username, Some(password));
+        }
+
+        let response = request.send().await?;
+        
+        if !response.status().is_success() {
+            return Err(anyhow!("Failed to query stored messages: {}", response.status()));
+        }
+
+        let result: serde_json::Value = response.json().await?;
+        let mut uids = Vec::new();
+
+        if let Some(rows) = result["rows"].as_array() {
+            for row in rows {
+                if let Some(id) = row["id"].as_str() {
+                    // Parse UID from document ID format: {mailbox}_{uid}
+                    if let Some(uid_str) = id.strip_prefix(&format!("{}_", mailbox)) {
+                        if let Ok(uid) = uid_str.parse::<u32>() {
+                            uids.push(uid);
+                        }
+                    }
+                }
+            }
+        }
+
+        Ok(uids)
+    }
+
     /// Delete a document (used in sync mode for deleted messages)
     pub async fn delete_document(&self, db_name: &str, doc_id: &str) -> Result<()> {
         // First get the document to get its revision