feat(matrixbot): stop auto-joining invites, restrict commands to two rooms

#general is now published to the public room directory, so the bot is
reachable by strangers. Two hardening changes:

- Invites are only accepted from INVITE_ALLOWED_USERS (defaults to
  ADMIN_USERS). Anything else is declined via room_leave so hostile
  invites do not accumulate as pending. Previously any invite from
  anyone was auto-joined.
- Commands and the passive scramble/riddle answer checks only run in
  COMMAND_ROOMS (#commands and #management). The gate sits early in
  Callbacks.message, so the bot stays silent everywhere else, including
  #general. Set COMMAND_ROOMS="*" to restore the old behaviour.

Reaction handlers are deliberately left ungated so the welcome flow
keeps working; game votes elsewhere cannot match an active game.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

matrixbot/bot.py

@@ -21,6 +21,7 @@ from config import (
     MATRIX_ACCESS_TOKEN,
     MATRIX_DEVICE_ID,
     MATRIX_PASSWORD,
+    INVITE_ALLOWED_USERS,
     LOG_LEVEL,
     ConfigValidator,
 )
@@ -148,13 +149,21 @@ async def main():
     client.add_event_callback(callbacks.unknown_event, UnknownEvent)
     client.add_event_callback(callbacks.member, RoomMemberEvent)
 
-    # Auto-accept room invites
-    async def _auto_accept_invite(room, event):
-        if event.membership == "invite" and event.state_key == MATRIX_USER_ID:
-            logger.info("Auto-accepting invite to %s", room.room_id)
-            await client.join(room.room_id)
+    # Accept invites only from trusted users, and decline the rest so they do
+    # not sit pending forever. Never auto-join on an arbitrary invite.
+    async def _handle_invite(room, event):
+        if event.membership != "invite" or event.state_key != MATRIX_USER_ID:
+            return
+        if event.sender not in INVITE_ALLOWED_USERS:
+            logger.warning(
+                "Declining invite to %s from untrusted sender %s", room.room_id, event.sender
+            )
+            await client.room_leave(room.room_id)
+            return
+        logger.info("Accepting invite to %s from %s", room.room_id, event.sender)
+        await client.join(room.room_id)
 
-    client.add_event_callback(_auto_accept_invite, InviteMemberEvent)
+    client.add_event_callback(_handle_invite, InviteMemberEvent)
 
     # Graceful shutdown
     loop = asyncio.get_running_loop()