fix: correct COPY paths in backend.prod.Dockerfile

The production backend Dockerfile was looking for Gradle files in a
backend/ subdirectory that doesn't exist in the repo structure:

- gradlew lives at repo root, not backend/gradlew
- gradle/ wrapper dir lives at repo root, not backend/gradle/
- settings.gradle lives at repo root, not backend/settings.gradle

Fixed by copying root-level Gradle files and placing backend-specific
files in the backend/ subdirectory. Also added :backend: subproject
prefix to Gradle tasks and corrected the output JAR path.

This fixes the deploy pipeline failure:
failed to calculate checksum: /backend/settings.gradle: not found

docker/backend.prod.Dockerfile

@@ -1,16 +1,17 @@
 FROM eclipse-temurin:21-jdk AS builder
 WORKDIR /app
-COPY backend/gradlew ./
-COPY backend/gradle/ ./gradle/
-COPY backend/build.gradle backend/settings.gradle ./
-RUN chmod +x gradlew && ./gradlew dependencies --no-daemon -q
-COPY backend/src ./src
-RUN ./gradlew bootJar --no-daemon -q
+COPY gradlew ./
+COPY gradle/ ./gradle/
+COPY settings.gradle ./
+COPY backend/build.gradle backend/
+RUN chmod +x gradlew && ./gradlew :backend:dependencies --no-daemon -q
+COPY backend/src backend/src
+RUN ./gradlew :backend:bootJar --no-daemon -q
 
 FROM eclipse-temurin:21-jre-alpine
 RUN addgroup -S bilhej && adduser -S bilhej -G bilhej
 WORKDIR /app
-COPY --from=builder /app/build/libs/*-SNAPSHOT.jar ./app.jar
+COPY --from=builder /app/backend/build/libs/*-SNAPSHOT.jar ./app.jar
 USER bilhej
 EXPOSE 8080
 ENTRYPOINT ["java", "-jar", "app.jar"]