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

frontend/src/api.ts

@@ -747,58 +747,3 @@ export async function testUpstreamSource(id: string): Promise<UpstreamSourceTest
   return handleResponse<UpstreamSourceTestResult>(response);
 }
 
-// =============================================================================
-// PyPI Cache Jobs API
-// =============================================================================
-
-import {
-  PyPICacheStatus,
-  PyPICacheTask,
-  PyPICacheActiveTask,
-  PyPICacheRetryResponse,
-} from './types';
-
-export async function getPyPICacheStatus(): Promise<PyPICacheStatus> {
-  const response = await fetch('/pypi/cache/status', {
-    credentials: 'include',
-  });
-  return handleResponse<PyPICacheStatus>(response);
-}
-
-export async function getPyPICacheFailedTasks(limit: number = 50): Promise<PyPICacheTask[]> {
-  const response = await fetch(`/pypi/cache/failed?limit=${limit}`, {
-    credentials: 'include',
-  });
-  return handleResponse<PyPICacheTask[]>(response);
-}
-
-export async function getPyPICacheActiveTasks(limit: number = 50): Promise<PyPICacheActiveTask[]> {
-  const response = await fetch(`/pypi/cache/active?limit=${limit}`, {
-    credentials: 'include',
-  });
-  return handleResponse<PyPICacheActiveTask[]>(response);
-}
-
-export async function retryPyPICacheTask(packageName: string): Promise<PyPICacheRetryResponse> {
-  const response = await fetch(`/pypi/cache/retry/${encodeURIComponent(packageName)}`, {
-    method: 'POST',
-    credentials: 'include',
-  });
-  return handleResponse<PyPICacheRetryResponse>(response);
-}
-
-export async function retryAllPyPICacheTasks(): Promise<PyPICacheRetryResponse> {
-  const response = await fetch('/pypi/cache/retry-all', {
-    method: 'POST',
-    credentials: 'include',
-  });
-  return handleResponse<PyPICacheRetryResponse>(response);
-}
-
-export async function cancelPyPICacheTask(packageName: string): Promise<PyPICacheRetryResponse> {
-  const response = await fetch(`/pypi/cache/cancel/${encodeURIComponent(packageName)}`, {
-    method: 'POST',
-    credentials: 'include',
-  });
-  return handleResponse<PyPICacheRetryResponse>(response);
-}