Add new fields to event theme model for asset handling

Added `background_image_url`, `foreground_image_url`, and `asset_image_urls` fields to enhance theme customization. Updated `asset_image_urls` to use `MutableDict` with a default empty dictionary and ensured consistency in the model and migration script.

backend/app/models/event_theme.py

@@ -1,4 +1,7 @@
+from typing import Dict
+
 from sqlalchemy import Column, String, JSON, Boolean
+from sqlalchemy.ext.mutable import MutableDict
 from sqlalchemy.orm import relationship
 from .base import Base, TimestampMixin, UUIDMixin
 
@@ -9,9 +12,14 @@ class EventTheme(Base, UUIDMixin, TimestampMixin):
     name = Column(String, nullable=False)
     description = Column(String)
     preview_image_url = Column(String)
+    background_image_url = Column(String, nullable=True)
+    foreground_image_url = Column(String, nullable=True)
+    asset_image_urls:  Dict[str, str] = Column(MutableDict.as_mutable(JSON), default=dict)
+
     color_palette = Column(JSON, nullable=False)
     fonts = Column(JSON, nullable=False)
     is_active = Column(Boolean, default=True, nullable=False)
+    
 
     # Relationship with Event
     events = relationship("Event", back_populates="theme")