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

app/matrix.py

@@ -0,0 +1,80 @@
+import logging
+
+from nio import AsyncClient, RoomSendResponse
+
+from app.config import config
+
+log = logging.getLogger(__name__)
+
+_client: AsyncClient | None = None
+
+THREAD_ROOT_MESSAGES: dict[str, str] = {
+    "movies": "\U0001f3ac Movies",
+    "shows": "\U0001f4fa Shows",
+    "music": "\U0001f3b5 Music",
+}
+
+
+async def get_client() -> AsyncClient:
+    global _client
+    if _client is None:
+        _client = AsyncClient(config.MATRIX_HOMESERVER_URL)
+        _client.user_id = config.MATRIX_BOT_USER_ID
+        _client.access_token = config.MATRIX_BOT_ACCESS_TOKEN
+    return _client
+
+
+async def close_client() -> None:
+    global _client
+    if _client is not None:
+        await _client.close()
+        _client = None
+
+
+async def send_thread_root(media_type: str) -> str:
+    client = await get_client()
+    label = THREAD_ROOT_MESSAGES[media_type]
+    content = {
+        "msgtype": "m.text",
+        "body": label,
+    }
+    resp = await client.room_send(
+        config.MATRIX_ARRIVALS_ROOM_ID,
+        "m.room.message",
+        content,
+    )
+    if isinstance(resp, RoomSendResponse):
+        log.info("Created thread root for %s: %s", media_type, resp.event_id)
+        return resp.event_id
+    raise RuntimeError(f"Failed to create thread root for {media_type}: {resp}")
+
+
+async def post_to_thread(
+    thread_root_event_id: str,
+    plain_body: str,
+    html_body: str,
+) -> str | None:
+    client = await get_client()
+    content = {
+        "msgtype": "m.text",
+        "format": "org.matrix.custom.html",
+        "body": plain_body,
+        "formatted_body": html_body,
+        "m.relates_to": {
+            "rel_type": "m.thread",
+            "event_id": thread_root_event_id,
+        },
+    }
+    try:
+        resp = await client.room_send(
+            config.MATRIX_ARRIVALS_ROOM_ID,
+            "m.room.message",
+            content,
+        )
+        if isinstance(resp, RoomSendResponse):
+            return resp.event_id
+        log.error("Matrix send failed: %s", resp)
+        return None
+    except Exception:
+        log.exception("Matrix posting error")
+        return None