feat(version): answer the build commit and time

GET /api/version is public. CI passes GIT_SHA and BUILD_TIME as Docker build args; a local build answers dev.

Dockerfile

@@ -25,6 +25,11 @@ FROM eclipse-temurin:25-jre-alpine
 
 WORKDIR /app
 
+# Passed by the CI workflow; shown by GET /api/version
+ARG GIT_SHA=dev
+ARG BUILD_TIME=
+ENV APP_BUILD_COMMIT=$GIT_SHA APP_BUILD_TIME=$BUILD_TIME
+
 # Expose the port your Spring Boot app runs on
 EXPOSE 8080