feat(ui): show build version in footer; enable watchtower for caddy

- Footer now displays build version tag + short commit SHA (from
  PUBLIC_BUILD_VERSION / PUBLIC_BUILD_COMMIT env vars baked in at
  image build time); falls back to 'dev' in local builds
- docker-compose.yml: add watchtower label to caddy service so it
  auto-updates alongside backend/ui/runner on new image pushes
- homelab/docker-compose.yml: use locally-built kokoro-fastapi:latest
  image (consistent with actual homelab setup)

docker-compose.yml

@@ -366,6 +366,8 @@ services:
     build:
       context: ./caddy
       dockerfile: Dockerfile
+    labels:
+      com.centurylinklabs.watchtower.enable: "true"
     restart: unless-stopped
     depends_on:
       backend: