Remove proactive PyPI dependency caching feature

The background task queue for proactively caching package dependencies was
causing server instability and unnecessary growth. The PyPI proxy now only
caches packages on-demand when users request them.

Removed:
- PyPI cache worker (background task queue and worker pool)
- PyPICacheTask model and related database schema
- Cache management API endpoints (/pypi/cache/*)
- Background Jobs admin dashboard
- Dependency extraction and queueing logic

Kept:
- On-demand package caching (still works when users request packages)
- Async httpx for non-blocking downloads (prevents health check failures)
- URL-based cache lookups for deduplication

backend/app/main.py

@@ -15,7 +15,6 @@ from .pypi_proxy import router as pypi_router
 from .seed import seed_database
 from .auth import create_default_admin
 from .rate_limit import limiter
-from .pypi_cache_worker import init_cache_worker_pool, shutdown_cache_worker_pool
 
 settings = get_settings()
 logging.basicConfig(level=logging.INFO)
@@ -50,14 +49,8 @@ async def lifespan(app: FastAPI):
     else:
         logger.info(f"Running in {settings.env} mode - skipping seed data")
 
-    # Initialize PyPI cache worker pool
-    init_cache_worker_pool()
-
     yield
 
-    # Shutdown: cleanup
-    shutdown_cache_worker_pool()
-
 
 app = FastAPI(
     title="Orchard",