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/simple-mail-document.json

@@ -0,0 +1,24 @@
+{
+  "_id": "Sent_456",
+  "_rev": "1-xyz789abc123def",
+  "sourceUid": "456",
+  "mailbox": "Sent",
+  "from": ["user@company.com"],
+  "to": ["client@external.com"],
+  "subject": "Meeting Follow-up",
+  "date": "2025-08-02T10:30:00Z",
+  "body": "Thank you for the productive meeting today. As discussed, I'll send the proposal by end of week.\n\nBest regards,\nUser Name",
+  "headers": {
+    "Content-Type": ["text/plain; charset=utf-8"],
+    "Content-Transfer-Encoding": ["7bit"],
+    "Date": ["Sat, 02 Aug 2025 12:30:00 +0200"],
+    "From": ["user@company.com"],
+    "To": ["client@external.com"],
+    "Subject": ["Meeting Follow-up"],
+    "Message-ID": ["<sent456.789@company.com>"],
+    "MIME-Version": ["1.0"]
+  },
+  "storedAt": "2025-08-02T12:30:45.123456789+02:00",
+  "docType": "mail",
+  "hasAttachments": false
+}