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/auth/require-admin.ts

@@ -0,0 +1,18 @@
+import { createClient } from '@/lib/supabase/server'
+import type { SupabaseClient } from '@supabase/supabase-js'
+import type { User } from '@supabase/supabase-js'
+
+// Shared guard for /api/admin/* routes: resolves the session and requires
+// the admin role. Returns user: null when the caller must respond 403.
+export async function requireAdmin(): Promise<{
+  supabase: SupabaseClient
+  user: User | null
+}> {
+  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 }
+}