av: clone broadcastbeat sources + AV Beat rebrand (Phase 1)

- Replaces the prior Rocket scaffold (saved on pre-bb-clone-rollback branch)
- Domain: broadcastbeat.com -> avbeat.com
- Brand: Broadcast Beat -> AV Beat
- Slug: broadcastbeat -> avbeat (package, identifiers)
- Schema fallback: bb -> av (env var name unchanged)
- Placeholder AV BEAT logo (gold->red gradient) at /assets/logos/av.svg
- All BB/RMP logo references repointed

Outstanding before public DNS swap:
  - Supabase av schema bootstrap (mirror of bb tables)
  - WordPress archive import (avbeat-com -> av.articles)
  - Coolify env vars
  - dev-avbeat.onsethost.com staging deploy + visual review

src/app/admin/newsletter/page.tsx

@@ -689,7 +689,7 @@ export default function AdminNewsletterPage() {
                   type="text"
                   value={digestSubject}
                   onChange={(e) => setDigestSubject(e.target.value)}
-                  placeholder="e.g. Broadcast Beat Weekly — NAB Show Highlights"
+                  placeholder="e.g. AV Beat Weekly — NAB Show Highlights"
                   className="search-input w-full py-2.5 px-3 text-sm"
                 />
               </div>