Add database schema with SQLAlchemy async + Alembic migrations

Set up PostgreSQL database layer with async SQLAlchemy 2.0 and asyncpg driver.
Implements 6 core tables (games, routes, pokemon, route_encounters, nuzlocke_runs,
encounters) with foreign keys, indexes, and an initial Alembic migration.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/app/core/config.py

@@ -14,8 +14,8 @@ class Settings(BaseSettings):
     # API settings
     api_v1_prefix: str = "/api/v1"
 
-    # Database settings (for future use)
-    database_url: str = "sqlite:///./nuzlocke.db"
+    # Database settings
+    database_url: str = "postgresql+asyncpg://postgres:postgres@localhost:5432/nuzlocke"
 
 
 settings = Settings()

backend/src/app/core/database.py

@@ -0,0 +1,31 @@
+from collections.abc import AsyncGenerator
+
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
+from sqlalchemy.orm import DeclarativeBase
+
+from app.core.config import settings
+
+
+def _get_async_url(url: str) -> str:
+    """Ensure the database URL uses the asyncpg driver."""
+    if url.startswith("postgresql://"):
+        return url.replace("postgresql://", "postgresql+asyncpg://", 1)
+    return url
+
+
+engine = create_async_engine(
+    _get_async_url(settings.database_url),
+    echo=settings.debug,
+    pool_pre_ping=True,
+)
+
+async_session = async_sessionmaker(engine, expire_on_commit=False)
+
+
+class Base(DeclarativeBase):
+    pass
+
+
+async def get_session() -> AsyncGenerator[AsyncSession]:
+    async with async_session() as session:
+        yield session