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/database.py

@@ -0,0 +1,85 @@
+import aiosqlite
+
+from app.config import config
+
+_db: aiosqlite.Connection | None = None
+
+
+async def get_db() -> aiosqlite.Connection:
+    global _db
+    if _db is None:
+        raise RuntimeError("Database not initialised — call init_db() first")
+    return _db
+
+
+async def init_db() -> None:
+    global _db
+    _db = await aiosqlite.connect(config.DATABASE_PATH)
+    _db.row_factory = aiosqlite.Row
+    await _db.execute(
+        """
+        CREATE TABLE IF NOT EXISTS thread_roots (
+            media_type TEXT PRIMARY KEY,
+            event_id   TEXT NOT NULL,
+            created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+        )
+        """
+    )
+    await _db.execute(
+        """
+        CREATE TABLE IF NOT EXISTS posted_events (
+            id         TEXT PRIMARY KEY,
+            title      TEXT NOT NULL,
+            media_type TEXT NOT NULL,
+            is_upgrade BOOLEAN NOT NULL,
+            posted_at  TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+        )
+        """
+    )
+    await _db.commit()
+
+
+async def close_db() -> None:
+    global _db
+    if _db is not None:
+        await _db.close()
+        _db = None
+
+
+async def get_thread_root(media_type: str) -> str | None:
+    db = await get_db()
+    cursor = await db.execute(
+        "SELECT event_id FROM thread_roots WHERE media_type = ?",
+        (media_type,),
+    )
+    row = await cursor.fetchone()
+    return row["event_id"] if row else None
+
+
+async def set_thread_root(media_type: str, event_id: str) -> None:
+    db = await get_db()
+    await db.execute(
+        "INSERT OR REPLACE INTO thread_roots (media_type, event_id) VALUES (?, ?)",
+        (media_type, event_id),
+    )
+    await db.commit()
+
+
+async def is_event_posted(event_key: str) -> bool:
+    db = await get_db()
+    cursor = await db.execute(
+        "SELECT 1 FROM posted_events WHERE id = ?",
+        (event_key,),
+    )
+    return await cursor.fetchone() is not None
+
+
+async def record_posted_event(
+    event_key: str, title: str, media_type: str, is_upgrade: bool
+) -> None:
+    db = await get_db()
+    await db.execute(
+        "INSERT OR IGNORE INTO posted_events (id, title, media_type, is_upgrade) VALUES (?, ?, ?, ?)",
+        (event_key, title, media_type, is_upgrade),
+    )
+    await db.commit()