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>

examples/sample-mail-document.json

@@ -0,0 +1,42 @@
+{
+  "_id": "INBOX_123",
+  "_rev": "1-abc123def456789",
+  "_attachments": {
+    "report.pdf": {
+      "content_type": "application/pdf",
+      "length": 245760,
+      "stub": true
+    },
+    "image.png": {
+      "content_type": "image/png", 
+      "length": 12345,
+      "stub": true
+    }
+  },
+  "sourceUid": "123",
+  "mailbox": "INBOX",
+  "from": ["sender@example.com", "alias@example.com"],
+  "to": ["recipient@company.com", "cc@company.com"],
+  "subject": "Monthly Report - Q3 2025",
+  "date": "2025-08-02T12:16:10Z",
+  "body": "Please find the attached monthly report for Q3 2025.\n\nBest regards,\nSender Name",
+  "headers": {
+    "Content-Type": ["multipart/mixed; boundary=\"----=_Part_123456\""],
+    "Content-Transfer-Encoding": ["7bit"],
+    "Date": ["Sat, 02 Aug 2025 14:16:10 +0200"],
+    "From": ["sender@example.com"],
+    "To": ["recipient@company.com"],
+    "Cc": ["cc@company.com"],
+    "Subject": ["Monthly Report - Q3 2025"],
+    "Message-ID": ["<msg123.456@example.com>"],
+    "MIME-Version": ["1.0"],
+    "X-Mailer": ["Mail Client 1.0"],
+    "Return-Path": ["<sender@example.com>"],
+    "Received": [
+      "from smtp.example.com (smtp.example.com [192.168.1.100]) by mx.company.com (Postfix) with ESMTP id ABC123; Sat, 02 Aug 2025 14:16:10 +0200"
+    ]
+  },
+  "storedAt": "2025-08-02T14:16:22.375241322+02:00",
+  "docType": "mail",
+  "hasAttachments": true
+}