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>

backend/pyproject.toml

@@ -21,6 +21,7 @@ dev = [
     "pytest>=8.0.0",
     "pytest-asyncio>=0.25.0",
     "httpx>=0.28.0",
+    "pokebase>=1.4.0",
 ]
 
 [build-system]