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>

Makefile

@@ -45,7 +45,11 @@ web-debug: $(PYTHON)
 	FLASK_DEBUG=1 $(PYTHON) app.py
 
 image:
-	docker build -t fuj-management:latest -f build/Dockerfile .
+	docker build -t fuj-management:latest \
+		--build-arg GIT_TAG=$$(git describe --tags --always 2>/dev/null || echo "untagged") \
+		--build-arg GIT_COMMIT=$$(git rev-parse --short HEAD) \
+		--build-arg BUILD_DATE=$$(date -u +%Y-%m-%dT%H:%M:%SZ) \
+		-f build/Dockerfile .
 
 run:
 	docker run -it --rm -p 5001:5001 -v $(CURDIR)/.secret:/app/.secret:ro fuj-management:latest