Phase 3: Schema Layer - Pydantic v2 migration, deduplication, and missing schemas

- Migrated all schemas to Pydantic v2 syntax (model_config, ConfigDict)
- Fixed mutable default in ProtoMessageBase using Field(default_factory=datetime.now)
- Consolidated CardCollection and Wishlist schemas in user_card_collection.py
- Created game_schemas.py with GameCreate, GameResponse, GameJoinRequest, etc.
- Created mtg_card_schemas.py with MtgCardResponse, MtgCardSearchRequest, etc.
- Added CardImportBatchCreate, CardImportBatchResponse, UserCardImportCreate/Response schemas
- Fixed duplicate UserCardImportRecord class between card_import_batch.py and user_card_import_record.py
- Updated __init__.py with comprehensive schema exports
- Created verify_schemas.py for schema-model matching verification

backend/app/models/user_deck.py

@@ -69,7 +69,7 @@ class UserDeckCard(Base):
 
     # Relationships
     deck = relationship("UserDeck", back_populates="cards")
-    card = relationship("MtgonlineCard", backref="deck_cards")
+    card = relationship("MtgonlineCard", back_populates="deck_cards")
 
     def __repr__(self) -> str:
         return f"<UserDeckCard deck={self.deck_id} card={self.card_id} qty={self.quantity}>"