feat(admin): add super admin initialization endpoint

Add POST /api/v1/admin/setup-super-admin endpoint to initialize the first super admin.
The endpoint can only be used once - after a super admin exists, subsequent requests
are rejected. Requires valid setup_secret configured via APP_SETUP_SECRET environment
variable or setup_secret in config file.

.gitea/workflows/deploy.yml

@@ -110,6 +110,7 @@ jobs:
               -e APP_GRPC_ENABLED=true \
               -e APP_GRPC_PORT=50051 \
               -e APP_SERVER_MODE=release \
+              -e "APP_SETUP_SECRET=${{ secrets.SETUP_SECRET }}" \
               ${IMAGE}
             
             echo "=== Waiting for startup ==="