riddle: add dedup cache to prevent repeated riddles

Keep a rolling list of the last 30 riddles used and inject them into
the prompt as an avoid clause, same pattern as trivia's per-category cache.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

matrixbot/commands.py

@@ -1762,14 +1762,21 @@ async def cmd_wyr(client: AsyncClient, room_id: str, sender: str, args: str):
 # ---------------------------------------------------------------------------
 
 _RIDDLE_ACTIVE: dict[str, dict] = {}
+_riddle_recent: list[str] = []
+_RIDDLE_RECENT_MAX = 30
 
 
 async def _generate_riddle() -> dict | None:
+    avoid_clause = (
+        " Do NOT use any of these riddles that were recently asked: "
+        + "; ".join(f'"{r}"' for r in _riddle_recent[-15:])
+        + "."
+    ) if _riddle_recent else ""
     system_msg = (
         "You are a riddle generator. Always respond with ONLY a JSON object — no markdown fences, no explanation. "
         'Format: {"riddle": "the riddle text", "answer": "short answer"}'
     )
-    user_msg = "Generate a clever riddle. The answer should be 1-4 words."
+    user_msg = f"Generate a clever, original riddle. The answer should be 1-4 words.{avoid_clause}"
     try:
         timeout = aiohttp.ClientTimeout(total=60)
         async with aiohttp.ClientSession(timeout=timeout) as session:
@@ -1795,6 +1802,9 @@ async def _generate_riddle() -> dict | None:
                 riddle = parsed.get("riddle", "").strip()
                 answer = parsed.get("answer", "").strip()
                 if riddle and answer:
+                    _riddle_recent.append(riddle)
+                    if len(_riddle_recent) > _RIDDLE_RECENT_MAX:
+                        _riddle_recent.pop(0)
                     return {"riddle": riddle, "answer": answer}
     except Exception as e:
         logger.error(f"riddle generation error: {e}", exc_info=True)