Add clear failed queue action

Add a Queue page bulk action and API route to remove all failed jobs, cover it with regression tests, and update the version, changelog, and README for the new queue workflow.

queue_jobs.py

@@ -1291,6 +1291,11 @@ class DownloadQueue:
             cursor = conn.execute("DELETE FROM jobs WHERE status IN ('done', 'canceled')")
             return {"ok": True, "count": cursor.rowcount}
 
+    def clear_failed(self):
+        with self.lock, self._connect() as conn:
+            cursor = conn.execute("DELETE FROM jobs WHERE status = 'failed'")
+            return {"ok": True, "count": cursor.rowcount}
+
     def _find(self, job_id):
         with self._connect() as conn:
             row = conn.execute("SELECT * FROM jobs WHERE id = ?", (job_id,)).fetchone()