feat(sprint7): Phase 4 — Integration (SMTP, tier enforcement, WebSocket)

Phase 4 implementation:
- 4.1 IONOS SMTP email configuration (production + docker profiles)
- 4.2 Portal navigation update (info board, events, forum links)
- 4.3 Tier enforcement: PlanTierService (forum=Pro+, info board limits)
- 4.4 WebSocket real-time updates (WebSocketEventPublisher)
- 4.5 EmailService: notification, event reminder, info board templates + rate limiting
- 4.6 Enterprise custom FROM: CustomMailDomain entity, DNS verification, controller

New files:
- PlanTierService: tier checks for forum/info board/enterprise features
- NotificationDispatchService: EMAIL channel dispatch via preferences
- WebSocketEventPublisher: STOMP topic push for forum/info board/events
- CustomMailDomainService: DNS TXT record verification for custom FROM
- MailSettingsController: Enterprise custom domain API endpoints
- CustomMailDomain entity + repository
- V16 migration: email dispatch index
- V17 migration: custom_mail_domains table
- Frontend: use-forum-subscription + use-info-board-subscription hooks
- Portal navbar: added info board, events, forum navigation items
- i18n: added portal nav translations (de + en)

Also fixed pre-existing Phase 2.5/3 compilation issues:
- Member entity: added userId field
- AuditService: added convenience overloads (logEvent, 4-param log)
- AuditEventType: added INFO_BOARD_POST_UPDATED, INFO_BOARD_POST_DELETED
- QuotaViolationCode: added TIER_UPGRADE_REQUIRED
- StaffPermissionChecker: added requirePermission(UserDetails, ...)
- TenantContext: added getCurrentTenantId() alias
- MemberRepository: added findByUserId, findByClubId, findAllByClubId
- EmailServiceTest: updated for new constructor signature

cannamanage-frontend/messages/de.json

@@ -243,6 +243,9 @@
     "networkError": "Verbindungsfehler. Bitte versuche es erneut.",
     "welcome": "Willkommen zurück, {name}!",
     "dashboard": "Übersicht",
+    "infoBoard": "Ankündigungen",
+    "events": "Termine",
+    "forum": "Forum",
     "quota": "Mein Kontingent",
     "history": "Ausgabe-Verlauf",
     "profile": "Profil",