fix(celery): preload every module's models so mapper resolution succeeds

Celery workers were failing every task that touched the DB with
InvalidRequestError: "expression 'ContentPage' failed to locate a name".

Root cause: the worker process only loads task modules during startup,
not routes or services. Models reached only via SQLAlchemy
string-based relationships (e.g. Platform.relationship("ContentPage"))
were never imported, so the mapper couldn't resolve the name when the
first task tried to open a DB session. The FastAPI process avoids this
because api_router transitively imports the world; the worker doesn't.

Add _preload_all_module_models() in celery_config.py that walks the
module registry and importlib.import_module's each "app.modules.<code>.models"
package. Called at module import time, before any task runs.

Surfaced while finally getting loyalty.send_notification_email registered
on the worker — the task ran, hit the DB to load email settings, and
exploded on the unresolved Platform -> ContentPage relationship.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/core/celery_config.py

@@ -64,6 +64,44 @@ def get_all_task_modules() -> list[str]:
     return []
 
 
+def _preload_all_module_models() -> None:
+    """
+    Import every module's models package up front.
+
+    The Celery worker process loads task modules but not routes/services,
+    so models reached only via SQLAlchemy string-based relationships
+    (e.g. Platform -> "ContentPage") never get imported. The first DB
+    query then explodes with InvalidRequestError when the mapper tries
+    to resolve the unknown name. The FastAPI process avoids this because
+    api_router transitively imports everything; we replicate that effect
+    here for the worker.
+    """
+    import importlib
+
+    try:
+        from app.modules.registry import MODULES
+    except ImportError as e:
+        logger.warning(f"Could not load module registry for model preload: {e}")
+        return
+
+    loaded = 0
+    for code in MODULES:
+        module_path = f"app.modules.{code}.models"
+        try:
+            importlib.import_module(module_path)
+            loaded += 1
+        except ModuleNotFoundError:
+            # Module doesn't expose a models package — fine, skip.
+            continue
+        except Exception as e:
+            logger.warning(f"Failed preloading {module_path}: {e}")
+
+    logger.info(f"Preloaded models for {loaded} modules (SQLAlchemy mapper resolution)")
+
+
+_preload_all_module_models()
+
+
 # Create Celery application
 celery_app = Celery(
     "orion",