Implement real Matrix notification for coe-mirror
Replace stub notify() with full Synapse integration targeting General room.
- Add _send_matrix(), _get_matrix_token(), _matrix_login() helpers
- Implement token cache with 30-min refresh threshold and 401 retry
- Fetch credentials via Infisical (MATRIX_USER, MATRIX_PASSWORD from matrix/)
- Add MATRIX_* env overrides for testing (homeserver, room, token cache path)
- Non-fatal failures: Matrix outage logs warning, does not break mirror run
- Add 5 comprehensive test cases covering success, failure, 401 refresh, env override, chmod 600
- All 18 tests passing
- Update README and notes.md with implementation details
n0mad1k
24d062fc5d