Add complete ai.card Rust implementation

- Implement complete Rust API server with axum framework
- Add database abstraction supporting PostgreSQL and SQLite
- Implement comprehensive gacha system with probability calculations
- Add JWT authentication with atproto DID integration
- Create card master data system with rarities (Normal, Rare, SuperRare, Kira, Unique)
- Implement draw history tracking and collection management
- Add API endpoints for authentication, card drawing, and collection viewing
- Include database migrations for both PostgreSQL and SQLite
- Maintain full compatibility with Python API implementation
- Add comprehensive documentation and development guide

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

python/api/app/repositories/user.py

@@ -0,0 +1,38 @@
+"""User repository"""
+from typing import Optional
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy.orm import selectinload
+
+from app.repositories.base import BaseRepository
+from app.db.models import User
+
+
+class UserRepository(BaseRepository[User]):
+    """User repository with custom methods"""
+    
+    def __init__(self, session: AsyncSession):
+        super().__init__(User, session)
+    
+    async def get_by_did(self, did: str) -> Optional[User]:
+        """Get user by DID"""
+        result = await self.session.execute(
+            select(User).where(User.did == did)
+        )
+        return result.scalar_one_or_none()
+    
+    async def get_or_create(self, did: str, handle: Optional[str] = None) -> User:
+        """Get existing user or create new one"""
+        user = await self.get_by_did(did)
+        if not user:
+            user = await self.create(did=did, handle=handle)
+        return user
+    
+    async def get_with_cards(self, user_id: int) -> Optional[User]:
+        """Get user with all their cards"""
+        result = await self.session.execute(
+            select(User)
+            .options(selectinload(User.cards))
+            .where(User.id == user_id)
+        )
+        return result.scalar_one_or_none()