refactor: address architect review findings (6 items)

R1: Extend @safe to catch ValueError->400, simplify routes_backtest
    (eliminated 4 copies of duplicated try/except)
R2: Consolidate PROVIDER constant into obb_utils.py (single source)
R3: Add days_ago() helper to obb_utils.py, replace 8+ duplications
R4: Extract Reddit/ApeWisdom into reddit_service.py from finnhub_service
R5: Fix missing top-level import asyncio in finnhub_service
R6: (deferred - sentiment logic extraction is a larger change)

All 561 tests passing.

route_utils.py

@@ -23,13 +23,19 @@ def validate_symbol(symbol: str) -> str:
 
 
 def safe(fn: Callable[P, Awaitable[R]]) -> Callable[P, Awaitable[R]]:
-    """Decorator to catch upstream errors and return 502."""
+    """Decorator to catch upstream errors and return 502.
+
+    ValueError is caught separately and returned as 400 (bad request).
+    All other non-HTTP exceptions become 502 (upstream error).
+    """
     @functools.wraps(fn)
     async def wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
         try:
             return await fn(*args, **kwargs)
         except HTTPException:
             raise
+        except ValueError as exc:
+            raise HTTPException(status_code=400, detail=str(exc))
         except Exception:
             logger.exception("Upstream data error")
             raise HTTPException(