fix: revert user_id filtering in task loaders to preserve backward compatibility

Avoid filtering loader queries by canonical user_id. Calling loaders with user_id=user_id introduces an exact-ID filter path that can drop valid legacy tasks: several loaders (e.g., load_due_market_trends_tasks) apply ...where task.user_id == user_id, but this commit also shifts discovery toward canonical IDs, so tasks persisted earlier with workspace-safe/sanitized IDs in the same per-user DB will no longer be returned and therefore never execute. Before this change, loaders were invoked as task_loader(db) and did not regress on mixed ID formats.

backend/services/scheduler/core/scheduler.py

@@ -700,14 +700,7 @@ class TaskScheduler:
             return summary
 
         try:
-            # Pass user_id for strict user isolation where loaders support it.
-            # Keep backward compatibility with legacy loaders that only accept db.
-            import inspect
-            loader_signature = inspect.signature(task_loader)
-            if "user_id" in loader_signature.parameters:
-                tasks = task_loader(db, user_id=user_id)
-            else:
-                tasks = task_loader(db)
+            tasks = task_loader(db)
 
             if not tasks:
                 return summary