feat(evidence): add S3 storage service and file_uploads model

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -47,6 +47,7 @@ from .notification_config import NotificationConfig
 from .notification_log import NotificationLog
 from .notification import Notification
 from .psa_activity_log import PsaActivityLog
+from .file_upload import FileUpload
 
 __all__ = [
     "User",
@@ -108,4 +109,5 @@ __all__ = [
     "NotificationLog",
     "Notification",
     "PsaActivityLog",
+    "FileUpload",
 ]