Address code review feedback: improve logging, fix redundant check, clarify docs

- Log exceptions at warning level in ConfigService (not debug)
- Include exc_info=True for startup seed failure logging
- Remove redundant `is_secret is not None` guard
- Clarify ★ markers in README environment variables docs

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>
Agent-Logs-Url: https://github.com/christianlouis/pop_puller_to_gmail/sessions/ce88d4a8-d8c2-49b3-95a1-30592105a769

backend/app/main.py

@@ -37,7 +37,7 @@ async def lifespan(app: FastAPI) -> AsyncIterator[None]:
         async with async_session_maker() as db:
             await ConfigService.seed_defaults(db)
     except Exception as exc:
-        logger.warning("Could not seed default settings: %s", exc)
+        logger.warning("Could not seed default settings: %s", exc, exc_info=True)
 
     yield
     # Shutdown