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

Dockerfile

@@ -17,6 +17,8 @@ COPY --from=deps /app/node_modules ./node_modules
 COPY . .
 
 ENV NEXT_TELEMETRY_DISABLED=1
+ARG NEXT_PUBLIC_APP_URL=http://localhost:3000
+ENV NEXT_PUBLIC_APP_URL=${NEXT_PUBLIC_APP_URL}
 RUN npm run build
 
 # --- Production image ---