This commit implements the full SSE real-time message push capability:
1. Add new `/api/messages/stream` SSE endpoint for real-time message delivery
2. Add MessageStreamer backend that polls DB with mtime awareness and broadcasts events
3. Add helper methods in DbReader: get_db_mtime and get_max_create_time
4. Optimize existing DB connections with `isolation_level=None`
5. Update bridge version to 1.5.0 and add `sse_push` to capability list
6. Add proper error handling and fallback logic for DB unavailability
