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

app/api/admin/users/route.ts

@@ -1,19 +1,9 @@
 export const dynamic = 'force-dynamic'
 
 import { NextRequest, NextResponse } from 'next/server'
-import { createClient } from '@/lib/supabase/server'
 import { createAdminClient } from '@/lib/supabase/admin'
 import { isValidRole } from '@/lib/auth/roles'
-
-async function requireAdmin() {
-  const supabase = await createClient()
-  const { data: { user }, error } = await supabase.auth.getUser()
-  if (error || !user) return { supabase, user: null }
-  const { data: profile } = await supabase
-    .from('users').select('role').eq('id', user.id).single()
-  if (!profile || profile.role !== 'admin') return { supabase, user: null }
-  return { supabase, user }
-}
+import { requireAdmin } from '@/lib/auth/require-admin'
 
 export async function GET() {
   const { supabase, user } = await requireAdmin()
@@ -108,6 +98,7 @@ export async function PATCH(request: NextRequest) {
 
   const { data: before } = await supabase
     .from('users').select('role, site_id, active, department').eq('id', body.id).single()
+  if (!before) return NextResponse.json({ error: 'User not found' }, { status: 404 })
 
   const { error } = await supabase.from('users').update(update).eq('id', body.id)
   if (error) return NextResponse.json({ error: 'Update failed' }, { status: 500 })