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.py

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+# backend/models/event.py
+from datetime import datetime, timezone
+from sqlalchemy import (
+    Column, String, DateTime, Time, Boolean, Integer, ForeignKey, JSON,
+    UniqueConstraint
+)
+from sqlalchemy.orm import relationship
+from sqlalchemy.dialects.postgresql import UUID
+from .base import Base, TimestampMixin, UUIDMixin
+
+
+class Event(Base, UUIDMixin, TimestampMixin):
+    __tablename__ = 'events'
+
+    # Basic Information
+    title = Column(String, nullable=False)
+    slug = Column(String, nullable=False)
+    description = Column(String)
+
+    # Location Details
+    location_name = Column(String)
+    location_address = Column(String)
+    location_url = Column(String)
+
+    # Timing - now timezone-aware
+    event_date = Column(DateTime(timezone=True), nullable=False)
+    event_start_time = Column(Time)
+    event_end_time = Column(Time)
+    timezone = Column(String, nullable=False)
+    rsvp_deadline = Column(DateTime(timezone=True))
+
+    # Rest of the fields remain the same...
+    is_public = Column(Boolean, default=False, nullable=False)
+    access_code = Column(String)
+    created_by = Column(UUID(as_uuid=True), ForeignKey('users.id'), nullable=False)
+    theme_id = Column(UUID(as_uuid=True), ForeignKey('event_themes.id'))
+    custom_theme_settings = Column(JSON)
+    additional_info = Column(JSON)
+    is_active = Column(Boolean, default=True, nullable=False)
+    rsvp_enabled = Column(Boolean, default=True, nullable=False)
+    gift_registry_enabled = Column(Boolean, default=True, nullable=False)
+    updates_enabled = Column(Boolean, default=True, nullable=False)
+    max_guests_per_invitation = Column(Integer)
+    contact_email = Column(String)
+    contact_phone = Column(String)
+
+    # Relationships remain the same...
+    creator = relationship("User", back_populates="created_events", foreign_keys=[created_by])
+    theme = relationship("EventTheme", back_populates="events")
+    managers = relationship("EventManager", back_populates="event")
+    guests = relationship("Guest", back_populates="event")
+    gift_items = relationship("GiftItem", back_populates="event")
+    media = relationship("EventMedia", back_populates="event")
+    updates = relationship("EventUpdate", back_populates="event")
+    rsvps = relationship("RSVP", back_populates="event")
+
+    __table_args__ = (
+        UniqueConstraint('slug', name='uq_event_slug'),
+    )
+
+    def __repr__(self):
+        return f"<Event {self.title} ({self.event_date})>"

backend/app/models/event_theme.py

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+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}>"