Add optional Matrix thread support for per-category deal organization

When MATRIX_USE_THREADS=true, deals are posted into dedicated threads
instead of the room timeline:

  - 🎮 Game Deals — CheapShark + ITAD type=game
  - 🧩 DLC Deals — ITAD type=dlc
  - 🆓 Epic Free Games — current and upcoming Epic free games
  - 📦 Non-Game Deals — ITAD non-game content (software, courses, etc.)

Thread roots are created on first use and persisted in the database.
When threads are disabled (default), behaviour is unchanged.

Key changes:
- New threads.py module with ThreadCategory enum and routing logic
- ITADDeal now carries deal_type; type filter moved from fetcher to bot
- MatrixDealsClient gains send_deal_in_thread() and create_thread_root()
- Database gets thread_roots table for storing root event IDs
- Bot routes each deal to the appropriate thread via _send_to_thread_or_room()

https://claude.ai/code/session_01EfPjktyrF24DBNHWsY1KBP

gaming_deals_bot/database.py

@@ -17,6 +17,12 @@ CREATE TABLE IF NOT EXISTS config (
     key TEXT PRIMARY KEY,
     value TEXT
 );
+
+CREATE TABLE IF NOT EXISTS thread_roots (
+    category TEXT PRIMARY KEY,
+    event_id TEXT NOT NULL,
+    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+);
 """
 
 
@@ -77,3 +83,33 @@ class Database:
             (key, value),
         )
         await self._db.commit()
+
+    # ------------------------------------------------------------------
+    # Thread root management
+    # ------------------------------------------------------------------
+
+    async def get_thread_root(self, category: str) -> str | None:
+        """Return the Matrix event ID for a thread root, or None."""
+        assert self._db is not None
+        cursor = await self._db.execute(
+            "SELECT event_id FROM thread_roots WHERE category = ?", (category,)
+        )
+        row = await cursor.fetchone()
+        return row[0] if row else None
+
+    async def set_thread_root(self, category: str, event_id: str):
+        """Store the Matrix event ID for a thread root."""
+        assert self._db is not None
+        await self._db.execute(
+            "INSERT OR REPLACE INTO thread_roots (category, event_id) VALUES (?, ?)",
+            (category, event_id),
+        )
+        await self._db.commit()
+
+    async def clear_thread_root(self, category: str):
+        """Remove a stored thread root (e.g. if the event was redacted)."""
+        assert self._db is not None
+        await self._db.execute(
+            "DELETE FROM thread_roots WHERE category = ?", (category,)
+        )
+        await self._db.commit()