refactor: address python review findings

- Move FRED credential registration to FastAPI lifespan (was fragile
  import-order-dependent side-effect)
- Add noqa E402 annotations for imports after curl_cffi patch
- Fix all return type hints: bare dict -> dict[str, Any]
- Move yfinance import to module level (was inline in functions)
- Fix datetime.now() -> datetime.now(tz=timezone.utc) in openbb_service
- Add try/except error handling to Group B service functions
- Fix dict mutation in relative_rotation (immutable pattern)
- Extract _classify_rrg_quadrant helper function
- Fix type builtin shadow in routes_economy (type -> gdp_type)
- Fix falsy int guard (if year: -> if year is not None:)
- Remove user input echo from error messages

quantitative_service.py

@@ -167,7 +167,7 @@ async def get_rolling_stat(
     """Compute a rolling statistic (variance, stdev, mean, skew, kurtosis, quantile)."""
     valid_stats = {"variance", "stdev", "mean", "skew", "kurtosis", "quantile"}
     if stat not in valid_stats:
-        return {"symbol": symbol, "error": f"Invalid stat: {stat}. Use: {', '.join(sorted(valid_stats))}"}
+        return {"symbol": symbol, "error": f"Invalid stat. Valid options: {', '.join(sorted(valid_stats))}"}
 
     fetch_days = max(days, PERF_DAYS)
     hist = await fetch_historical(symbol, fetch_days)