Add game data seeding from PokeAPI with level ranges

Seed the database with Pokemon game data for 5 games (FireRed, LeafGreen,
Emerald, HeartGold, SoulSilver) using pokebase. Includes Alembic migrations
for route unique constraints and encounter level ranges, a two-phase seed
system (offline fetch to JSON, then idempotent upserts), and Dockerfile
updates for the seed runner.

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

docker-compose.yml

@@ -7,6 +7,7 @@ services:
       - "8000:8000"
     volumes:
       - ./backend/src:/app/src:cached
+      - ./backend/alembic.ini:/app/alembic.ini:cached
     environment:
       - DEBUG=true
       - DATABASE_URL=postgresql://postgres:postgres@db:5432/nuzlocke