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/threads.py

@@ -0,0 +1,59 @@
+"""Thread category definitions for Matrix room threads.
+
+When ``MATRIX_USE_THREADS=true``, deal messages are posted inside
+per-category threads rather than directly into the room timeline.
+"""
+
+from enum import Enum
+from html import escape
+
+
+class ThreadCategory(str, Enum):
+    """Categories that map to distinct Matrix threads."""
+
+    GAME_DEALS = "game_deals"
+    DLC_DEALS = "dlc_deals"
+    EPIC_FREE = "epic_free"
+    NON_GAME_DEALS = "non_game_deals"
+
+
+# Display metadata for each thread category.
+# ``label`` is the thread root title; ``description`` appears below it.
+THREAD_META: dict[ThreadCategory, dict[str, str]] = {
+    ThreadCategory.GAME_DEALS: {
+        "label": "🎮 Game Deals",
+        "description": "PC game deals from CheapShark and IsThereAnyDeal",
+    },
+    ThreadCategory.DLC_DEALS: {
+        "label": "🧩 DLC Deals",
+        "description": "DLC and expansion deals from IsThereAnyDeal",
+    },
+    ThreadCategory.EPIC_FREE: {
+        "label": "🆓 Epic Free Games",
+        "description": "Weekly free games from the Epic Games Store",
+    },
+    ThreadCategory.NON_GAME_DEALS: {
+        "label": "📦 Non-Game Deals",
+        "description": "Software, courses, and other non-game deals",
+    },
+}
+
+
+def itad_type_to_category(deal_type: str) -> ThreadCategory:
+    """Map an ITAD ``type`` value to a thread category."""
+    if deal_type == "game":
+        return ThreadCategory.GAME_DEALS
+    if deal_type == "dlc":
+        return ThreadCategory.DLC_DEALS
+    return ThreadCategory.NON_GAME_DEALS
+
+
+def format_thread_root(category: ThreadCategory) -> tuple[str, str]:
+    """Return ``(plain_text, html)`` for a thread root message."""
+    meta = THREAD_META[category]
+    label = meta["label"]
+    desc = meta["description"]
+
+    plain_text = f"{label}\n{desc}"
+    html = f"<strong>{escape(label)}</strong><br>\n<em>{escape(desc)}</em>"
+    return plain_text, html