feat: add worker threads, persistent Python sidecar, graceful shutdown, and architectural improvements

- Graceful shutdown: SIGTERM/SIGINT handlers drain HTTP, stop workers, close DB
- Thumbnail caching: disk-cached thumbnails with immutable Cache-Control headers
- Worker thread pool: Piscina offloads Sharp processing off the main event loop
- Persistent Python dispatcher: pre-imports ML libraries, eliminates cold-start latency
- Tool page registry: declarative tool-to-component mapping replaces 750-line switch
- File store cleanup: remove dead derived fields, stable files array reference
- Job persistence: progress written to SQLite jobs table, stale jobs recovered on startup

apps/api/package.json

@@ -30,6 +30,7 @@
     "jsqr": "^1.4.0",
     "p-queue": "^9.1.0",
     "pdfkit": "^0.18.0",
+    "piscina": "^5.1.4",
     "potrace": "^2.1.8",
     "qrcode": "^1.5.4",
     "sharp": "^0.33.0",