refactor(logging): use logger.exception to preserve stack traces

Unify error logging across services: replace logger.error(f"...{e}")
and logger.error(..., exc_info=True) with logger.exception() so the
stack trace is consistently captured.

bot/services/stars_service.py

@@ -55,7 +55,7 @@ class StarsService:
                 title=description,
                 description=description,
                 payload=payload,
-                provider_token="",
+                provider_token="",  # Required to be empty for Telegram Stars (XTR) per Telegram Bot API.
                 currency="XTR",
                 prices=prices,
             )