refactor: consolidate 10+ model files into 2 modules

- models.py: All application models (users, decks, sessions, groups, imports, etc.)
- mtg_models.py: All MTG card data models (sets, cards, mirrors, links)
- Removed: mirror_models.py, user_data.py, user_deck.py, card_import_batch.py,
  user_card_import.py, user_card_import_record.py, relationships.py
- Added __backup__/ directory with original files for reference
- DecklistFile.card_links cross-model relationship resolved via late import

backend/app/models/mtg_models.py

@@ -1,14 +1,29 @@
 """
-SQLAlchemy ORM models for the MTG database (mtgjson.com data).
+SQLAlchemy ORM models for MTG card data and mirrors.
 
-Models mirror the mtg_cards and mtg_sets tables in the MTG PostgreSQL database.
+Consolidated from mtg_models.py and mirror_models.py into a single module.
+
+This file contains all MTG card data models:
+- MtgSet, MtgCard: Canonical card/set data from mtgjson.com (mtg_data DB)
+- MtgCardMirror: Mirrored card data copied to mtgonline DB for fast deckbuilding queries
+- DeckCardLink: Junction table linking user decks to mirrored cards
+
+Cross-model references to application models (e.g., DeckCardLink.deck → DecklistFile)
+use string-based relationship() calls to avoid circular imports.
 """
-from sqlalchemy import Column, Integer, String, Text, DateTime, ForeignKey, Index
+from sqlalchemy import (
+    Column, Integer, String, Text, DateTime, ForeignKey, Index,
+    UniqueConstraint,
+)
 from sqlalchemy.orm import relationship
 from sqlalchemy.sql import func
 from app.core.database import Base
 
 
+# ---------------------------------------------------------------------------
+# MTG Set and Card (canonical data from mtgjson.com)
+# ---------------------------------------------------------------------------
+
 class MtgSet(Base):
     """MTG Set model."""
     __tablename__ = "mtg_sets"
@@ -65,7 +80,105 @@ class MtgCard(Base):
         return f"<MtgCard {self.name} ({self.set_id})>"
 
 
+# ---------------------------------------------------------------------------
+# Card Mirror and Deck Links (mtgonline DB)
+# ---------------------------------------------------------------------------
+
+class MtgCardMirror(Base):
+    """
+    Mirrored card data for user decks.
+
+    Contains all relevant card fields copied from mtg_cards (mtg_data)
+    to avoid cross-DB joins during deckbuilding. Back-references source_id
+    to the canonical mtg_cards.id for sync tracking.
+    """
+    __tablename__ = "mtg_cards_mirror"
+
+    id = Column(Integer, primary_key=True, index=True)
+    source_id = Column(
+        Integer,
+        nullable=True,
+        index=True,
+        comment="Logical reference to mtg_cards.id in mtg_data database "
+                "(cross-DB reference, no FK constraint)",
+    )
+    name = Column(String(255), nullable=False, index=True)
+    mana_cost = Column(String(255), nullable=True)
+    type_line = Column(String(255), nullable=True)
+    oracle_text = Column(Text, nullable=True)
+    power = Column(String(50), nullable=True)
+    toughness = Column(String(50), nullable=True)
+    rarity = Column(String(50), nullable=True)
+    layout = Column(String(50), nullable=True)
+    artist = Column(String(255), nullable=True)
+    flavor_text = Column(Text, nullable=True)
+    numbers = Column(String(100), nullable=True)
+    identifiers = Column(Text, nullable=True)  # JSON string of all identifiers
+    images = Column(Text, nullable=True)  # JSON string of image URLs
+    image = Column(Text, nullable=True)  # Card image URL from MTGJSON
+    card_parts = Column(Text, nullable=True)  # Comma-separated list of face names
+    keywords = Column(Text, nullable=True)  # Comma-separated keywords
+    legalities = Column(Text, nullable=True)  # JSON of format legality
+    set_code = Column(String(10), nullable=True, index=True)
+    set_name = Column(String(255), nullable=True)
+    # Sync tracking
+    synced_at = Column(DateTime, server_default=func.now(), onupdate=func.now())
+    created_at = Column(DateTime, server_default=func.now())
+
+    # Relationships
+    deck_links = relationship(
+        "DeckCardLink",
+        back_populates="card",
+        cascade="all, delete-orphan",
+    )
+
+    def __repr__(self) -> str:
+        return f"<MtgCardMirror {self.name} (ID: {self.id}, source: {self.source_id})>"
+
+
+class DeckCardLink(Base):
+    """
+    Junction table linking user decks to mirrored cards.
+
+    Represents a specific quantity of a mirrored card in a specific zone
+    (main deck or sideboard) of a user's deck.
+    """
+    __tablename__ = "deck_card_links"
+
+    id = Column(Integer, primary_key=True, index=True)
+    deck_id = Column(
+        Integer, ForeignKey("mtgonline_decklist_files.id", ondelete="CASCADE"),
+        nullable=False,
+    )
+    card_id = Column(
+        Integer, ForeignKey("mtg_cards_mirror.id"), nullable=False,
+    )
+    quantity = Column(Integer, nullable=False, default=1)
+    zone = Column(String(20), nullable=False, default="main")  # 'main' or 'sideboard'
+
+    # Composite unique: a card can only appear once per zone in a deck
+    __table_args__ = (
+        UniqueConstraint("deck_id", "card_id", "zone", name="uq_deck_card_link"),
+        Index("idx_deck_card_deck", "deck_id"),
+        Index("idx_deck_card_card", "card_id"),
+    )
+
+    # Relationships
+    # String reference to DecklistFile in models.py to avoid circular imports
+    deck = relationship("DecklistFile", back_populates="card_links")
+    card = relationship("MtgCardMirror", back_populates="deck_links")
+
+    def __repr__(self) -> str:
+        return (
+            f"<DeckCardLink deck={self.deck_id} card={self.card_id} "
+            f"qty={self.quantity} zone={self.zone}>"
+        )
+
+
+# ---------------------------------------------------------------------------
 # Indexes for performance
+# ---------------------------------------------------------------------------
+
 Index("idx_mtg_cards_name_set", MtgCard.name, MtgCard.set_id)
 Index("idx_mtg_cards_type", MtgCard.type_line)
 Index("idx_mtg_cards_rarity", MtgCard.rarity)