Add sync error logging, extend error flash, fix unload saves

- Log workforce sync errors to backend stdout so they appear in docker logs
- Extend error flash from 5s to 15s so errors are readable
- Replace sendBeacon (POST-only) with keepalive fetch (PUT) on unload —
  sendBeacon was hitting 404s on all three unload saves

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/node_modules/ajv/lib/vocabularies/metadata.ts

@@ -0,0 +1,17 @@
+import type {Vocabulary} from "../types"
+
+export const metadataVocabulary: Vocabulary = [
+  "title",
+  "description",
+  "default",
+  "deprecated",
+  "readOnly",
+  "writeOnly",
+  "examples",
+]
+
+export const contentVocabulary: Vocabulary = [
+  "contentMediaType",
+  "contentEncoding",
+  "contentSchema",
+]