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/pages/api/jobs/[id]/[action].ts

@@ -34,6 +34,19 @@ export const POST: APIRoute = async ({ params, locals }) => {
       for (const it of items) await enqueue({ itemId: it.id, jobId: id });
       break;
     }
+    case 'delete': {
+      // Auftrag samt Positionen und deren Objekten entfernen (für alte/fehlgeschlagene Aufträge).
+      const { deleteObject } = await import('../../../../lib/storage');
+      const items = await query<any>(
+        `SELECT source_path, source_paths, result_path, thumb_path FROM items WHERE job_id=$1`, [id]);
+      for (const it of items) {
+        const keys = [it.source_path, it.result_path, it.thumb_path, ...((it.source_paths as string[]) || [])].filter(Boolean);
+        for (const k of keys) await deleteObject(k).catch(() => {});
+      }
+      await query(`DELETE FROM items WHERE job_id=$1`, [id]);
+      await query(`DELETE FROM jobs WHERE id=$1`, [id]);
+      break;
+    }
     default:
       return json({ error: 'Unbekannte Aktion.' }, 400);
   }