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-domain/src/main/java/de/cannamanage/domain/entity/Notification.java

@@ -0,0 +1,64 @@
+package de.cannamanage.domain.entity;
+
+import de.cannamanage.domain.enums.NotificationType;
+import jakarta.persistence.*;
+
+import java.util.UUID;
+
+/**
+ * Persistent notification entity — survives page refresh.
+ * Delivered in real-time via WebSocket, but also queryable via REST.
+ */
+@Entity
+@Table(name = "notifications", indexes = {
+    @Index(name = "idx_notifications_user", columnList = "user_id, read, created_at DESC")
+})
+public class Notification extends AbstractTenantEntity {
+
+    @Column(name = "user_id", nullable = false)
+    private UUID userId;
+
+    @Enumerated(EnumType.STRING)
+    @Column(name = "type", nullable = false, length = 50)
+    private NotificationType type;
+
+    @Column(name = "title", nullable = false)
+    private String title;
+
+    @Column(name = "message", nullable = false, columnDefinition = "TEXT")
+    private String message;
+
+    @Column(name = "link", length = 500)
+    private String link;
+
+    @Column(name = "read", nullable = false)
+    private boolean read = false;
+
+    public Notification() {}
+
+    public Notification(UUID userId, NotificationType type, String title, String message, String link) {
+        this.userId = userId;
+        this.type = type;
+        this.title = title;
+        this.message = message;
+        this.link = link;
+    }
+
+    public UUID getUserId() { return userId; }
+    public void setUserId(UUID userId) { this.userId = userId; }
+
+    public NotificationType getType() { return type; }
+    public void setType(NotificationType type) { this.type = type; }
+
+    public String getTitle() { return title; }
+    public void setTitle(String title) { this.title = title; }
+
+    public String getMessage() { return message; }
+    public void setMessage(String message) { this.message = message; }
+
+    public String getLink() { return link; }
+    public void setLink(String link) { this.link = link; }
+
+    public boolean isRead() { return read; }
+    public void setRead(boolean read) { this.read = read; }
+}

cannamanage-domain/src/main/java/de/cannamanage/domain/enums/NotificationType.java

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+package de.cannamanage.domain.enums;
+
+/**
+ * Types of notifications sent to users.
+ */
+public enum NotificationType {
+    QUOTA_WARNING,
+    BATCH_RECALLED,
+    DISTRIBUTION_RECORDED,
+    SUBSCRIPTION_EXPIRING
+}