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/relationships.py

@@ -0,0 +1,24 @@
+"""
+Cross-model relationships.
+
+This module defines relationships that reference models from other
+model files (e.g., DecklistFile.card_links → DeckCardLink). These
+must be defined AFTER all model classes have been imported to avoid
+circular import issues.
+
+Import this module LAST in app/models/__init__.py.
+"""
+from sqlalchemy.orm import relationship
+from app.models.models import DecklistFile
+from app.models.mirror_models import DeckCardLink
+
+
+# Add the back-reference from DecklistFile to DeckCardLink.
+# This was previously defined dynamically at the bottom of mirror_models.py,
+# but that caused circular imports. Now it lives here and is imported last.
+DecklistFile.card_links = relationship(
+    "DeckCardLink",
+    back_populates="deck",
+    cascade="all, delete-orphan",
+    order_by="DeckCardLink.id"
+)