Add Event and EventTheme models

Introduces models for events and their themes, including fields for event details, location, timing, and settings. The `EventTheme` model supports theme customization with color palettes and fonts, while establishing relationships with the `Event` model. These additions prepare the database for event-related functionality.

backend/app/models/event_theme.py

@@ -0,0 +1,19 @@
+from sqlalchemy import Column, String, JSON
+from sqlalchemy.orm import relationship
+from .base import Base, TimestampMixin, UUIDMixin
+
+
+class EventTheme(Base, UUIDMixin, TimestampMixin):
+    __tablename__ = 'event_themes'
+
+    name = Column(String, nullable=False)
+    description = Column(String)
+    preview_image_url = Column(String)
+    color_palette = Column(JSON, nullable=False)
+    fonts = Column(JSON, nullable=False)
+
+    # Relationship with Event
+    events = relationship("Event", back_populates="theme")
+
+    def __repr__(self):
+        return f"<EventTheme {self.name}>"