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-sync-metadata.json

@@ -0,0 +1,10 @@
+{
+  "_id": "sync_metadata_INBOX",
+  "_rev": "2-def456abc789123",
+  "docType": "sync_metadata",
+  "mailbox": "INBOX",
+  "lastSyncTime": "2025-08-02T14:26:08.281094+02:00",
+  "lastMessageUID": 123,
+  "messageCount": 45,
+  "updatedAt": "2025-08-02T14:26:08.281094+02:00"
+}