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was revoked/expired: our stats API collapsed both "Lichess rejected the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.

LichessWebServices/main.py

@@ -704,7 +704,10 @@ async def get_puzzle_of_period(
     try:
         result = await stats_service.get_puzzle_of_period(token, since, until, max)
         if not result.success:
-            # Реальная ошибка при обращении к Lichess — не маскируем её под "0 пазлов"
+            if result.auth_failed:
+                # Токен отклонён Lichess (401/403) — permanent, ретраить бессмысленно
+                raise HTTPException(status_code=401, detail=result.message)
+            # Прочая (транзитная) ошибка при обращении к Lichess — не маскируем её под "0 пазлов"
             raise HTTPException(status_code=502, detail=result.message)
         return result
     except HTTPException: