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/validation/const.ts

@@ -0,0 +1,28 @@
+import type {CodeKeywordDefinition, ErrorObject, KeywordErrorDefinition} from "../../types"
+import type {KeywordCxt} from "../../compile/validate"
+import {_} from "../../compile/codegen"
+import {useFunc} from "../../compile/util"
+import equal from "../../runtime/equal"
+
+export type ConstError = ErrorObject<"const", {allowedValue: any}>
+
+const error: KeywordErrorDefinition = {
+  message: "must be equal to constant",
+  params: ({schemaCode}) => _`{allowedValue: ${schemaCode}}`,
+}
+
+const def: CodeKeywordDefinition = {
+  keyword: "const",
+  $data: true,
+  error,
+  code(cxt: KeywordCxt) {
+    const {gen, data, $data, schemaCode, schema} = cxt
+    if ($data || (schema && typeof schema == "object")) {
+      cxt.fail$data(_`!${useFunc(gen, equal)}(${data}, ${schemaCode})`)
+    } else {
+      cxt.fail(_`${schema} !== ${data}`)
+    }
+  },
+}
+
+export default def