feat: complete deckbuilding feature with local card mirror

- Add MtgonlineCard model (local card data mirror in mtgonline DB)
- Create user_deck.py models (UserDeck, UserDeckCard, DeckPrecedent, CardSuggestion)
- Create user_deck_schemas.py with Pydantic schemas
- Update decks.py router to use local MtgonlineCard instead of cross-DB MtgCard
- Add migration 002 (user deck building tables)
- Add migration 003 (mtgonline_cards table)
- All card lookups now use local mirror for fast queries
- No cross-DB joins in deckbuilding endpoints

backend/alembic/env.py

@@ -24,6 +24,7 @@ from app.models.user_data import (
     UserGroup, GroupMember, GroupChatMessage, UserNetwork,
     NetworkMember, UserPreference, UserActivityLog
 )
+from app.models.user_deck import UserDeck, UserDeckCard, DeckPrecedent, DeckPrecedentCard, CardSuggestion
 
 # this is the Alembic Config object
 config = context.config