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/pyproject.toml

@@ -10,6 +10,9 @@ dependencies = [
     "pydantic>=2.10.0",
     "pydantic-settings>=2.7.0",
     "python-dotenv>=1.0.0",
+    "sqlalchemy[asyncio]>=2.0.0",
+    "asyncpg>=0.30.0",
+    "alembic>=1.14.0",
 ]
 
 [project.optional-dependencies]