stop infinite retry loop on rejected Lichess tokens

A gamer's periodic check would get permanently stuck if their stored token
was revoked/expired: our stats API collapsed both "Lichess rejected the
token" (401/403, permanent) and genuine transient errors into the same 502
response, so the bot treated an invalid token exactly like a network blip —
retrying the same window forever at a capped 300s backoff, never advancing
the checkpoint (observed in prod: Dor1zz stuck for 100+ consecutive errors
over 8+ hours, admin alerts firing every 25 failures).

Preserve the distinction that already existed one layer down (lichess_client.py
already tells 401/403 apart from other failures) instead of collapsing it in
stats_service.py: add PuzzleOfPeriodResponse.auth_failed, have main.py return
401 specifically for that case, and have the bot raise a distinct
InvalidTokenError instead of returning None. On InvalidTokenError, the bot now
clears the token for that pair, notifies the user to reconnect via /addtoken,
and continues tracking games normally instead of stalling forever.

LichessClientTG_bot/database.py

@@ -335,6 +335,16 @@ class Database:
             )
             conn.commit()
 
+    def clear_user_gamer_token(self, user_id: int, gamer_id: int):
+        """Clear a rejected/expired token so periodic checks stop sending it (games-only monitoring continues)."""
+        with sqlite3.connect(self.db_path) as conn:
+            cursor = conn.cursor()
+            cursor.execute(
+                "UPDATE user_gamers SET token = NULL WHERE user_id = ? AND gamer_id = ?",
+                (user_id, gamer_id)
+            )
+            conn.commit()
+
     def get_period_checkpoint(self, user_id: int, gamer_id: int) -> Optional[int]:
         """Get persisted period checkpoint timestamp (seconds since epoch)."""
         with sqlite3.connect(self.db_path) as conn: