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/jtd/metadata.ts

@@ -0,0 +1,24 @@
+import {KeywordCxt} from "../../ajv"
+import type {CodeKeywordDefinition} from "../../types"
+import {alwaysValidSchema} from "../../compile/util"
+
+const def: CodeKeywordDefinition = {
+  keyword: "metadata",
+  schemaType: "object",
+  code(cxt: KeywordCxt) {
+    checkMetadata(cxt)
+    const {gen, schema, it} = cxt
+    if (alwaysValidSchema(it, schema)) return
+    const valid = gen.name("valid")
+    cxt.subschema({keyword: "metadata", jtdMetadata: true}, valid)
+    cxt.ok(valid)
+  },
+}
+
+export function checkMetadata({it, keyword}: KeywordCxt, metadata?: boolean): void {
+  if (it.jtdMetadata !== metadata) {
+    throw new Error(`JTD: "${keyword}" cannot be used in this schema location`)
+  }
+}
+
+export default def