Pairing code auth, replace Facebook OAuth, public feed

- Add PairingCode model, POST /generate + /redeem + GET /status endpoints
- Remove facebookId from User, make metaId non-nullable
- Delete meta-web routes, link routes, meta-web-auth service
- Remove metaWeb config block and hasFacebookLink from responses
- Add optionalAuth middleware, make feed publicly accessible
- Resolve Twitch channel names for embed broadcastIds

prisma/schema.prisma

@@ -9,8 +9,7 @@ datasource db {
 
 model User {
   id          String   @id @default(uuid())
-  metaId      String?  @unique
-  facebookId  String?  @unique
+  metaId      String   @unique
   displayName String
   email       String?
   avatarUrl   String?
@@ -22,11 +21,23 @@ model User {
   linkedAccounts LinkedAccount[]
   streamPlans    StreamPlan[]
   sessions       Session[]
+  pairingCodes   PairingCode[]
   followers      Follow[]        @relation("following")
   following      Follow[]        @relation("follower")
   likes          Like[]
 }
 
+model PairingCode {
+  id        String   @id @default(uuid())
+  code      String   @unique
+  userId    String
+  user      User     @relation(fields: [userId], references: [id], onDelete: Cascade)
+  expiresAt DateTime
+  createdAt DateTime @default(now())
+
+  @@index([userId])
+}
+
 model Session {
   id           String   @id @default(uuid())
   userId       String