Implement Melora: *arr media arrival webhook announcer for Matrix

Replace Bellhop scaffolding with full Melora implementation:
- FastAPI webhook endpoints for Radarr, Sonarr, and Lidarr
- matrix-nio integration with threaded room posting
- SQLite for thread root persistence and event deduplication
- Message formatting with plain text and HTML for each media type
- Shared secret authentication via X-Arr-Webhook-Secret header
- Updated dependencies, configuration, and documentation

https://claude.ai/code/session_01DuzWyMMXvLMB4VxEwJyV4X

.gitignore

@@ -1,6 +1,6 @@
 __pycache__/
 *.pyc
 .env
-bellhop.db
+melora.db
 *.sqlite
 *.sqlite3