fix: code-review findings — closed-incident embedding 503, hardening, dedup

- /api/incidents/[id]/similar: embedding backfill on a closed incident hit
  the closure-lock trigger and turned the whole request into a 503; now
  skips persistence for closed incidents (vector still used for the query)
- addenda: cap body at 5000 chars; include body text in audit_log entry
- admin users PATCH: 404 when target user does not exist (was silent ok)
- extract shared requireAdmin to lib/auth/require-admin.ts (was duplicated
  in admin users + sites routes)
- extract escapeCsv/rowsToCsv to lib/csv.ts (was duplicated in dashboard
  export route and lib/reports/jkkp8.ts)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CPf5Rc8QPx6V8KLEEgfKEQ

lib/reports/jkkp8.ts

@@ -1,4 +1,5 @@
 import { computeDoshObligation } from '@/lib/incidents/dosh'
+import { escapeCsv } from '@/lib/csv'
 
 export interface Jkkp8Incident {
   reference_no: string | null
@@ -73,15 +74,6 @@ export const JKKP8_HEADERS = [
   'Description', 'Medical Status', 'Lost Days', 'NADOPOD Obligation', 'DOSH Filing Status',
 ]
 
-export function escapeCsv(value: string | number | null | undefined): string {
-  if (value === null || value === undefined) return ''
-  const str = String(value)
-  if (str.includes(',') || str.includes('"') || str.includes('\n')) {
-    return `"${str.replace(/"/g, '""')}"`
-  }
-  return str
-}
-
 export function jkkp8Csv(rows: Jkkp8Row[]): string {
   const lines = [JKKP8_HEADERS.map(escapeCsv).join(',')]
   for (const r of rows) {