fix: Critical security fixes and code cleanup for Turso PR

Security Fixes:
- Limit gRPC message size to 64MB (was usize::MAX) - CRITICAL DoS fix
- Add HTTP body size limits (10MB) to admin and user APIs
- Restrict CORS to specific methods/headers (was Any)

Code Cleanup:
- Delete 8 temporary files (CI_TRIGGER.txt, MIGRATION_*.md, etc.)
- Remove 90 lines of commented Connection struct code
- Remove commented // mod h2c and allocator lines
- Run cargo fmt for formatting

Files changed:
- replication_log_proxy.rs: Add MAX_DECODING_MESSAGE_SIZE constant
- admin/mod.rs: Add DefaultBodyLimit import and 10MB limit
- user/mod.rs: Add DefaultBodyLimit import, CORS restrictions, 10MB limit
- lib.rs: Remove commented code
- connection.rs: Remove 90 lines of commented code

Author: devonshigaki