Rewrite fetch_pokeapi.py to read from local submodule instead of PokeAPI

Replace all pokebase API calls with local JSON file reads from the
PokeAPI/api-data git submodule, removing the network dependency and
pokebase package. The script now runs with stdlib Python only.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/pyproject.toml

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