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

backend/app/pypi_cache_worker.py

@@ -526,6 +526,38 @@ def get_failed_tasks(db: Session, limit: int = 50) -> List[dict]:
     ]
 
 
+def get_active_tasks(db: Session, limit: int = 50) -> List[dict]:
+    """
+    Get list of currently active (in_progress) tasks.
+
+    Args:
+        db: Database session.
+        limit: Maximum number of tasks to return.
+
+    Returns:
+        List of active task info dicts.
+    """
+    tasks = (
+        db.query(PyPICacheTask)
+        .filter(PyPICacheTask.status == "in_progress")
+        .order_by(PyPICacheTask.started_at.desc())
+        .limit(limit)
+        .all()
+    )
+
+    return [
+        {
+            "id": str(task.id),
+            "package": task.package_name,
+            "version_constraint": task.version_constraint,
+            "depth": task.depth,
+            "attempts": task.attempts,
+            "started_at": task.started_at.isoformat() if task.started_at else None,
+        }
+        for task in tasks
+    ]
+
+
 def retry_failed_task(db: Session, package_name: str) -> Optional[PyPICacheTask]:
     """
     Reset a failed task to retry.