docs: add comprehensive CouchDB schema documentation for cross-implementation compatibility

- Add complete CouchDB document schema specifications in couchdb-schemas.md
- Create example JSON documents for mail and sync metadata structures
- Implement Rust schema definitions with full serde support and type safety
- Add validation script to ensure schema consistency across implementations
- Document field definitions, data types, and validation rules
- Provide Rust Cargo.toml with appropriate dependencies for future implementation

This establishes a solid foundation for the planned Rust implementation while ensuring
100% compatibility with existing Go implementation databases. Both implementations will
use identical document structures, field names, and database naming conventions.

Schema Features:
- Mail documents with native CouchDB attachment support
- Sync metadata for incremental synchronization
- Predictable document ID patterns for efficient access
- Cross-language type mappings and validation rules
- Example documents for testing and reference

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

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

rust/Cargo.toml

@@ -0,0 +1,52 @@
+[package]
+name = "mail2couch"
+version = "0.1.0"
+edition = "2021"
+description = "A powerful email backup utility that synchronizes mail from IMAP accounts to CouchDB"
+license = "MIT"
+repository = "https://github.com/yourusername/mail2couch"
+keywords = ["email", "backup", "imap", "couchdb", "sync"]
+categories = ["email", "database"]
+
+[dependencies]
+# Serialization
+serde = { version = "1.0", features = ["derive"] }
+serde_json = "1.0"
+
+# Date/time handling
+chrono = { version = "0.4", features = ["serde"] }
+
+# HTTP client for CouchDB
+reqwest = { version = "0.11", features = ["json"] }
+
+# Async runtime
+tokio = { version = "1.0", features = ["full"] }
+
+# Error handling
+thiserror = "1.0"
+anyhow = "1.0"
+
+# Configuration
+config = "0.13"
+
+# IMAP client (when implementing IMAP functionality)
+# async-imap = "0.9"  # Commented out for now due to compatibility issues
+
+# Logging
+log = "0.4"
+env_logger = "0.10"
+
+# CLI argument parsing
+clap = { version = "4.0", features = ["derive"] }
+
+[dev-dependencies]
+# Testing utilities
+tokio-test = "0.4"
+
+[lib]
+name = "mail2couch"
+path = "src/lib.rs"
+
+[[bin]]
+name = "mail2couch"
+path = "src/main.rs"