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/src/main.rs

@@ -0,0 +1,7 @@
+// Placeholder main.rs for Rust implementation
+// This will be implemented in the future
+
+fn main() {
+    println!("mail2couch Rust implementation - Coming Soon!");
+    println!("See the Go implementation in ../go/ for current functionality.");
+}