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/models/__init__.py

@@ -0,0 +1,15 @@
+from app.models.encounter import Encounter
+from app.models.game import Game
+from app.models.nuzlocke_run import NuzlockeRun
+from app.models.pokemon import Pokemon
+from app.models.route import Route
+from app.models.route_encounter import RouteEncounter
+
+__all__ = [
+    "Encounter",
+    "Game",
+    "NuzlockeRun",
+    "Pokemon",
+    "Route",
+    "RouteEncounter",
+]