feat(sprint-5): Phase 1 — Docker Compose full stack, CORS, Next.js upgrade

- Dockerfile.backend: multi-stage Java 21 build (eclipse-temurin)
- docker-compose.yml: PostgreSQL 16 + backend + frontend with health checks
- SecurityConfig: CORS for localhost:3000 frontend origin
- application-docker.properties: Docker profile with env vars
- Spring Boot Actuator health endpoint enabled
- Next.js upgraded 15.2.8 → 15.5.18 (security fixes)

cannamanage-api/pom.xml

@@ -118,6 +118,11 @@
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-starter-mail</artifactId>
         </dependency>
+        <!-- Actuator (health endpoint for Docker) -->
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-actuator</artifactId>
+        </dependency>
     </dependencies>
 
     <build>