feat: Bake build metadata (git tag, commit, date) into OCI image and display in web UI

Store git tag, commit hash, and build date as OCI-standard labels and a
build_meta.json file inside the Docker image. The Flask app reads this at
startup and displays version info in all page footers. Adds /version JSON
endpoint for programmatic access. Falls back to dev@local outside Docker.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

build/Dockerfile

@@ -24,6 +24,17 @@ COPY templates/ ./templates/
 COPY build/entrypoint.sh /entrypoint.sh
 RUN chmod +x /entrypoint.sh
 
+ARG GIT_TAG=unknown
+ARG GIT_COMMIT=unknown
+ARG BUILD_DATE=unknown
+
+LABEL org.opencontainers.image.version="${GIT_TAG}" \
+      org.opencontainers.image.revision="${GIT_COMMIT}" \
+      org.opencontainers.image.created="${BUILD_DATE}" \
+      org.opencontainers.image.title="fuj-management"
+
+RUN echo "{\"tag\": \"${GIT_TAG}\", \"commit\": \"${GIT_COMMIT}\", \"build_date\": \"${BUILD_DATE}\"}" > /app/build_meta.json
+
 EXPOSE 5001
 
 HEALTHCHECK --interval=60s --timeout=5s --start-period=5s \