feat: management polish, !cancel, !wordlestats, welcome fixes

- Add !cancel command (anyone cancels own blackjack; PL50+ clears all room games)
- Add !wordlestats top-level command (wraps wordle stats function)
- Add !cleanwelcome admin command to purge stale welcome DM records
- !help now hides management section from sub-PL50 users, hides !health from non-admins
- !announce uses nio room cache for join_rule instead of an API call per room
- Fix _INVITEALL_BLOCKED comment (Commands is knock-gated, not restricted)
- welcome.py: skip duplicate DM if a pending welcome already exists for the user
- welcome.py: add clean_stale_dm_messages() helper
- welcome.py: replace no-op post_welcome_message with log_ready()
- bot.py: update import/call to match welcome.py rename

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

matrixbot/bot.py

@@ -26,7 +26,7 @@ from config import (
 )
 from callbacks import Callbacks
 from utils import setup_logging
-from welcome import post_welcome_message
+from welcome import log_ready as _welcome_log_ready
 
 logger = setup_logging(LOG_LEVEL)
 
@@ -179,8 +179,7 @@ async def main():
     # Trust devices after initial sync loads the device store
     await trust_devices(client)
 
-    # Post welcome message (idempotent — only posts if not already stored)
-    await post_welcome_message(client)
+    _welcome_log_ready()
 
     logger.info("Bot ready as %s — listening for commands", MATRIX_USER_ID)