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/incidents/[id]/addenda/route.ts

@@ -41,6 +41,8 @@ export async function POST(
   const body: { body?: string } = await request.json().catch(() => ({}))
   const text = (body.body ?? '').trim()
   if (!text) return NextResponse.json({ error: 'body required' }, { status: 422 })
+  if (text.length > 5000)
+    return NextResponse.json({ error: 'body must be 5000 characters or fewer' }, { status: 422 })
 
   const { data: addendum, error } = await supabase
     .from('incident_addenda')
@@ -54,7 +56,7 @@ export async function POST(
     p_table_name: 'incident_addenda',
     p_record_id: addendum.id,
     p_action: 'INSERT',
-    p_new_value: { incident_id: id, author: user.id },
+    p_new_value: { incident_id: id, author: user.id, body: text },
   })
 
   return NextResponse.json({ id: addendum.id }, { status: 201 })