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

requirements.txt

@@ -1,9 +1,6 @@
 fastapi==0.115.6
 uvicorn[standard]==0.34.0
-httpx==0.28.1
 aiosqlite==0.20.0
 python-dotenv==1.0.1
 matrix-nio==0.24.0
-slowapi==0.1.9
-jinja2==3.1.5
-itsdangerous==2.2.0
+markdown==3.7