Add Active Workers table to Background Jobs dashboard

Shows currently processing cache tasks in a dynamic table with:
- Package name and version constraint being cached
- Recursion depth and attempt number
- Start timestamp
- Pulsing indicator to show live activity

Backend changes:
- Add get_active_tasks() function to pypi_cache_worker.py
- Add GET /pypi/cache/active endpoint to pypi_proxy.py

Frontend changes:
- Add PyPICacheActiveTask type
- Add getPyPICacheActiveTasks() API function
- Add Active Workers section with animated table
- Auto-refreshes every 5 seconds with existing data

frontend/src/api.ts

@@ -754,6 +754,7 @@ export async function testUpstreamSource(id: string): Promise<UpstreamSourceTest
 import {
   PyPICacheStatus,
   PyPICacheTask,
+  PyPICacheActiveTask,
   PyPICacheRetryResponse,
 } from './types';
 
@@ -771,6 +772,13 @@ export async function getPyPICacheFailedTasks(limit: number = 50): Promise<PyPIC
   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',