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]/similar/route.ts

@@ -22,12 +22,12 @@ export async function GET(
 
   const { data: incident } = await supabase
     .from('incidents')
-    .select('id, description, embedding')
+    .select('id, description, embedding, status')
     .eq('id', id)
     .single()
   if (!incident) return NextResponse.json({ error: 'Not found' }, { status: 404 })
 
-  const inc = incident as { id: string; description: string; embedding: string | null }
+  const inc = incident as { id: string; description: string; embedding: string | null; status: string }
 
   let embeddingVec: number[]
   try {
@@ -35,9 +35,14 @@ export async function GET(
       embeddingVec = JSON.parse(inc.embedding) as number[]
     } else {
       embeddingVec = await embedText(inc.description, voyageKey)
-      await supabase.from('incidents').update({
-        embedding: `[${embeddingVec.join(',')}]` as unknown as string,
-      }).eq('id', id)
+      // Closed incidents are locked at the DB level — the trigger would reject
+      // this backfill. The vector still serves the similarity query below.
+      if (inc.status !== 'closed') {
+        const { error: persistError } = await supabase.from('incidents').update({
+          embedding: `[${embeddingVec.join(',')}]` as unknown as string,
+        }).eq('id', id)
+        if (persistError) console.error('embedding backfill error:', persistError)
+      }
     }
   } catch {
     return NextResponse.json({ error: 'Embedding service unavailable' }, { status: 503 })