feat: add invite code system and multi-image Drone pipeline

- Add InviteCode model with code, maxUses, expiry, usage tracking
- Registration now requires a valid invite code
- New users get USER role instead of ADMIN
- Admin-only /invites page to create, manage, and share invite codes
- Invite links auto-fill code via ?code= URL param
- Drone pipeline now builds app, worker, and bot images separately
- Add NEXT_PUBLIC_APP_URL build arg to fix URL redirects

prisma/schema.prisma

@@ -38,6 +38,7 @@ model User {
   tags         Tag[]
   settings     UserSettings?
   telegramLink TelegramLink?
+  inviteCodes  InviteCode[]
 }
 
 model Account {
@@ -554,6 +555,21 @@ model GlobalSetting {
   @@map("global_settings")
 }
 
+model InviteCode {
+  id        String    @id @default(cuid())
+  code      String    @unique @db.VarChar(32)
+  maxUses   Int       @default(1)
+  uses      Int       @default(0)
+  expiresAt DateTime?
+  createdBy String
+  createdAt DateTime  @default(now())
+
+  creator User @relation(fields: [createdBy], references: [id], onDelete: Cascade)
+
+  @@index([code])
+  @@map("invite_codes")
+}
+
 model ChannelFetchRequest {
   id         String      @id @default(cuid())
   accountId  String