feat(sprint7): Phase 3 — Forum MVP

- Flyway V15: forum_topics, forum_replies, forum_reactions, forum_reports tables
- Enums: ForumTargetType, ReactionType, ReportStatus
- Extended AuditEventType with FORUM_REPLY_CREATED, FORUM_REPORT_REVIEWED
- Entities: ForumTopic, ForumReply, ForumReaction, ForumReport
- Repositories: ForumTopicRepository, ForumReplyRepository, ForumReactionRepository, ForumReportRepository
- ForumService: full CRUD, moderation (lock/pin/delete), 60-min edit window,
  toggle reactions, content reporting, notifications on new topics/replies
- ForumController: admin + portal endpoints (topics, replies, reactions, reports, moderation)
- Frontend: forum.ts service with React Query hooks (admin + portal)
- Frontend: Admin forum page with topic list, moderation actions (lock/pin/delete)
- Frontend: Portal forum page with topic list, reply thread, reactions, report
- Navigation: added Forum with MessageSquare icon
- i18n: forum.* keys in de.json and en.json

cannamanage-frontend/messages/de.json

@@ -808,5 +808,33 @@
       "WORKSHOP": "Workshop",
       "OTHER": "Sonstiges"
     }
+  },
+  "forum": {
+    "title": "Forum",
+    "description": "Vereinsinternes Diskussionsforum",
+    "newTopic": "Neues Thema",
+    "topicTitlePlaceholder": "Titel des Themas...",
+    "topicContentPlaceholder": "Beschreibe dein Thema...",
+    "creating": "Wird erstellt...",
+    "create": "Erstellen",
+    "cancel": "Abbrechen",
+    "loading": "Wird geladen...",
+    "noTopics": "Noch keine Themen vorhanden. Erstelle das erste!",
+    "replies": "Antworten",
+    "lastReply": "Letzte Antwort",
+    "openReports": "offene Meldungen",
+    "pin": "Anheften",
+    "unpin": "Lösen",
+    "lock": "Sperren",
+    "unlock": "Entsperren",
+    "delete": "Löschen",
+    "deleteReason": "Grund für die Löschung (optional):",
+    "replyPlaceholder": "Schreibe eine Antwort...",
+    "sending": "Wird gesendet...",
+    "reply": "Antworten",
+    "edited": "bearbeitet",
+    "topicLocked": "Dieses Thema ist gesperrt. Neue Antworten sind nicht möglich.",
+    "reportReason": "Grund der Meldung:",
+    "backToTopics": "Zurück zur Übersicht"
   }
 }