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/config.py

@@ -62,6 +62,11 @@ class Config:
         )
         self.itad_deals_limit = int(os.environ.get("ITAD_DEALS_LIMIT", "200"))
 
+        # Matrix threads — post deals into per-category threads
+        self.matrix_use_threads = os.environ.get(
+            "MATRIX_USE_THREADS", "false"
+        ).lower() in ("true", "1", "yes")
+
         # Intro message on startup
         self.send_intro_message = os.environ.get(
             "SEND_INTRO_MESSAGE", "false"