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()