devops(backend): pin eclipse-temurin tags, skip test compilation, document jar glob

- Pin to eclipse-temurin:21.0.10_7-{jdk,jre}-noble for reproducible builds
- Switch -DskipTests to -Dmaven.test.skip=true: skips test compilation entirely,
  not just execution — faster and avoids build failures from test-only missing classes
- Add comment on COPY *.jar explaining why the glob is safe (Spring Boot renames
  the pre-repackage artifact to .jar.original, leaving only one .jar in target/)

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

backend/Dockerfile

@@ -1,4 +1,4 @@
-FROM eclipse-temurin:21-jdk AS builder
+FROM eclipse-temurin:21.0.10_7-jdk-noble AS builder
 WORKDIR /app
 
 # Copy wrapper and POM first — dependency layer is cached separately from source
@@ -7,10 +7,12 @@ COPY mvnw pom.xml ./
 RUN --mount=type=cache,target=/root/.m2 ./mvnw dependency:go-offline -q
 
 COPY src ./src
-RUN --mount=type=cache,target=/root/.m2 ./mvnw clean package -DskipTests -q
+# -Dmaven.test.skip=true skips test compilation entirely (not just execution)
+RUN --mount=type=cache,target=/root/.m2 ./mvnw clean package -Dmaven.test.skip=true -q
 
-FROM eclipse-temurin:21-jre
+FROM eclipse-temurin:21.0.10_7-jre-noble
 WORKDIR /app
+# Spring Boot repackages to *.jar; pre-repackage artifact uses .jar.original, not .jar
 COPY --from=builder /app/target/*.jar app.jar
 EXPOSE 8080
 CMD ["java", "-jar", "app.jar"]