feat(sprint-6): Phase 6 — Notifications (WebSocket) + PWA

- WebSocket: Spring STOMP + SockJS, NotificationService, persistent notifications table
- NotificationController: GET/PUT endpoints for notification management
- Frontend: notification bell with unread badge, dropdown panel, real-time via STOMP
- PWA: manifest.json, service worker (manual sw.js), offline page, install prompt
- PWA icons (192+512), dark theme colors, standalone display
- Full i18n (de/en) for notifications and PWA
- Flyway V10 migration for notifications table
- spring-boot-starter-websocket dependency added

cannamanage-service/src/main/java/de/cannamanage/service/NotificationService.java

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+package de.cannamanage.service;
+
+import de.cannamanage.domain.entity.Notification;
+import de.cannamanage.domain.enums.NotificationType;
+import de.cannamanage.service.repository.NotificationRepository;
+import lombok.RequiredArgsConstructor;
+import lombok.extern.slf4j.Slf4j;
+import org.springframework.messaging.simp.SimpMessagingTemplate;
+import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Transactional;
+
+import java.util.List;
+import java.util.Map;
+import java.util.UUID;
+
+/**
+ * Notification service — persists notifications and delivers in real-time via WebSocket.
+ */
+@Slf4j
+@Service
+@RequiredArgsConstructor
+public class NotificationService {
+
+    private final NotificationRepository notificationRepository;
+    private final SimpMessagingTemplate messagingTemplate;
+
+    /**
+     * Send a notification: persist to DB + push via WebSocket.
+     */
+    @Transactional
+    public Notification sendNotification(UUID userId, NotificationType type, String title, String message, String link) {
+        var notification = new Notification(userId, type, title, message, link);
+        notification = notificationRepository.save(notification);
+
+        // Push to user's WebSocket queue
+        messagingTemplate.convertAndSendToUser(
+                userId.toString(),
+                "/queue/notifications",
+                Map.of(
+                        "id", notification.getId(),
+                        "type", notification.getType().name(),
+                        "title", notification.getTitle(),
+                        "message", notification.getMessage(),
+                        "link", notification.getLink() != null ? notification.getLink() : "",
+                        "read", notification.isRead(),
+                        "createdAt", notification.getCreatedAt().toString()
+                )
+        );
+
+        log.debug("Notification sent to user {}: {} - {}", userId, type, title);
+        return notification;
+    }
+
+    /**
+     * Get the last 10 notifications for a user (unread first).
+     */
+    @Transactional(readOnly = true)
+    public List<Notification> getRecentNotifications(UUID userId) {
+        return notificationRepository.findTop10ByUserIdOrderByReadAscCreatedAtDesc(userId);
+    }
+
+    /**
+     * Get unread count for badge display.
+     */
+    @Transactional(readOnly = true)
+    public long getUnreadCount(UUID userId) {
+        return notificationRepository.countByUserIdAndReadFalse(userId);
+    }
+
+    /**
+     * Mark a single notification as read.
+     */
+    @Transactional
+    public void markAsRead(UUID notificationId) {
+        notificationRepository.findById(notificationId).ifPresent(n -> {
+            n.setRead(true);
+            notificationRepository.save(n);
+        });
+    }
+
+    /**
+     * Mark all notifications as read for a user.
+     */
+    @Transactional
+    public int markAllAsRead(UUID userId) {
+        return notificationRepository.markAllAsRead(userId);
+    }
+}

cannamanage-service/src/main/java/de/cannamanage/service/repository/NotificationRepository.java

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+package de.cannamanage.service.repository;
+
+import de.cannamanage.domain.entity.Notification;
+import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.data.jpa.repository.Modifying;
+import org.springframework.data.jpa.repository.Query;
+import org.springframework.data.repository.query.Param;
+import org.springframework.stereotype.Repository;
+
+import java.util.List;
+import java.util.UUID;
+
+@Repository
+public interface NotificationRepository extends JpaRepository<Notification, UUID> {
+
+    List<Notification> findByUserIdOrderByReadAscCreatedAtDesc(UUID userId);
+
+    List<Notification> findTop10ByUserIdOrderByReadAscCreatedAtDesc(UUID userId);
+
+    long countByUserIdAndReadFalse(UUID userId);
+
+    @Modifying
+    @Query("UPDATE Notification n SET n.read = true WHERE n.userId = :userId AND n.read = false")
+    int markAllAsRead(@Param("userId") UUID userId);
+}