Add validation for display_order and EventMedia tests

Introduce a `validate_display_order` method to ensure a default value of 0 for `display_order`. Extend testing infrastructure with fixtures and unit tests for `EventMedia` to validate properties, metadata, and default behaviors.

backend/app/models/event_media.py

@@ -1,7 +1,7 @@
 from enum import Enum
 from sqlalchemy import Column, String, Integer, Boolean, ForeignKey, JSON, Enum as SQLEnum
 from sqlalchemy.dialects.postgresql import UUID
-from sqlalchemy.orm import relationship
+from sqlalchemy.orm import relationship, validates
 from .base import Base, TimestampMixin, UUIDMixin
 
 
@@ -48,6 +48,11 @@ class EventMedia(Base, UUIDMixin, TimestampMixin):
     def __repr__(self):
         return f"<EventMedia {self.original_filename} ({self.media_type}) for event={self.event_id}>"
 
+    @validates('display_order')
+    def validate_display_order(self, key, value):
+        return value if value is not None else 0
+
+
     @property
     def is_image(self):
         return self.media_type == MediaType.IMAGE