Show app version in admin UI and /version endpoint

Embed APP_VERSION build arg in Docker image (default: dev).
build_push.sh passes the git tag as build arg. Proxy exposes
GET /version, admin UI shows it as read-only field in settings.

Dockerfile

@@ -6,6 +6,8 @@ COPY frontend/ frontend/
 RUN npm run build --prefix frontend
 
 FROM python:3.12-slim
+ARG APP_VERSION=dev
+ENV APP_VERSION=$APP_VERSION
 WORKDIR /app
 
 COPY backend/requirements.txt .