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was revoked/expired: our stats API collapsed both "Lichess rejected the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

@@ -253,6 +253,7 @@ class PuzzleOfPeriodResponse(BaseModel):
     """
     message: str = Field(..., description="Сообщение о результате запроса", example="Статистика решения задач за период")
     success: bool = Field(True, description="False, если запрос к Lichess завершился ошибкой (а не легитимным нулевым результатом)", example=True)
+    auth_failed: bool = Field(False, description="True, если Lichess отклонил токен (401/403) — permanent, не стоит ретраить; отличается от транзитных ошибок")
     period_start: int = Field(..., description="Начало периода (Unix timestamp в миллисекундах)", example=1640995200000)
     period_end: int = Field(..., description="Конец периода (Unix timestamp в миллисекундах)", example=1641081600000)
     max_puzzles: int = Field(..., description="Максимальное количество задач для получения", example=50)