feat: llms.txt (AI-readable docs), job delete, friendly missing-source error

- /llms.txt: public machine-readable capability/API/MCP doc for agents.
- Job delete action + queue button for terminal jobs; cleans objects.
- process.ts: clear 'Quelle nicht mehr vorhanden' message instead of raw
  EISDIR/ENOENT when a source was purged.
- Docs corrected: Picdrop 'missing images' was web-UI sorting, not PNG;
  JPG delivery kept as size/perf improvement.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XNQ8ghPfzAfsyVYd6HgFb6

src/components/QueueApp.tsx

@@ -78,6 +78,13 @@ export default function QueueApp({ jobId }: { jobId: string | null }) {
           {job?.status === 'paused' && <button className="mini" onClick={() => act(`/api/jobs/${jobId}/resume`)}>Fortsetzen</button>}
           {failed > 0 && <button className="mini" onClick={() => act(`/api/jobs/${jobId}/retry-failed`)}>Fehlgeschlagene erneut</button>}
           {['queued', 'running', 'paused'].includes(job?.status) && <button className="mini" onClick={() => act(`/api/jobs/${jobId}/cancel`)}>Abbrechen</button>}
+          {['done', 'failed', 'cancelled'].includes(job?.status) && (
+            <button className="mini" onClick={async () => {
+              if (!confirm('Diesen Auftrag löschen?')) return;
+              await fetch(`/api/jobs/${jobId}/delete`, { method: 'POST' });
+              location.href = '/warteschlange';
+            }}>Löschen</button>
+          )}
         </div>
       </div>