feat(avbeat): point forum + auth at av schema, swap newsletter chips to AV verticals

Schema wiring — AV Beat was inheriting a 'bb' default schema from the
original fork. Fixed three call sites so the AV Beat app reads/writes
the av schema unless an explicit NEXT_PUBLIC_SUPABASE_SCHEMA override
is set:
  - src/lib/supabase/client.tsx:8 default 'bb' -> 'av'
  - src/lib/supabase/server.tsx:11 default 'bb' -> 'av'
  - src/app/api/auth/lookup-email/route.ts :31 + :42 hardcoded
    .schema('bb') -> .schema('av') so signup lookup hits the
    AV-side subscriber tables, not BB's.

Newsletter signup — homepage chips were the BB taxonomy ('Broadcast',
'Post Production', 'Animation', 'AI & Automation', 'Live Production',
'NAB Show'). Swapped for pro-AV verticals that align with the new
forum categories: Conference Rooms & UC, Digital Signage, Live Events
& Staging, Displays & LED, Audio, AV over IP, InfoComm & ISE.
Default selected flipped from 'Broadcast' to 'Conference Rooms & UC'
so the form doesn't pre-select a non-existent value.

src/lib/supabase/client.tsx

@@ -5,7 +5,7 @@ export function createClient() {
     process.env.NEXT_PUBLIC_SUPABASE_URL!,
     process.env.NEXT_PUBLIC_SUPABASE_ANON_KEY!,
     {
-      db: { schema: process.env.NEXT_PUBLIC_SUPABASE_SCHEMA || 'bb' },
+      db: { schema: process.env.NEXT_PUBLIC_SUPABASE_SCHEMA || 'av' },
       cookies: {
         getAll() {
           if (typeof document === 'undefined') return [];

src/lib/supabase/server.tsx

@@ -8,7 +8,7 @@ export async function createClient() {
     process.env.NEXT_PUBLIC_SUPABASE_URL!,
     process.env.NEXT_PUBLIC_SUPABASE_ANON_KEY!,
     {
-      db: { schema: process.env.NEXT_PUBLIC_SUPABASE_SCHEMA || 'bb' },
+      db: { schema: process.env.NEXT_PUBLIC_SUPABASE_SCHEMA || 'av' },
       cookies: {
         getAll() {
           return cookieStore.getAll();