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/main.py

@@ -1,14 +1,25 @@
+from contextlib import asynccontextmanager
+
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
 from app.api.routes import api_router
 from app.core.config import settings
+from app.core.database import engine
+
+
+@asynccontextmanager
+async def lifespan(app: FastAPI):
+    yield
+    await engine.dispose()
+
 
 app = FastAPI(
     title=settings.app_name,
     openapi_url=f"{settings.api_v1_prefix}/openapi.json",
     docs_url="/docs",
     redoc_url="/redoc",
+    lifespan=lifespan,
 )
 
 # Configure CORS