feat(B-NOTIF): Notification→Message Konsolidierung — System-Channel, post_system_message(), Migration

B-NOTIF-SYS: CommConversation um is_system Feld erweitert, get_or_create_system_channel()
B-NOTIF-EVT: post_system_message() in kommunikation/services.py — erstellt CommMessage im System-Channel
- create_notification() als deprecated Wrapper delegiert auf post_system_message()
- Text-Block + action_card Block mit Deep-Link, Metadata: notification_type/severity/entity_ref
B-NOTIF-PREF: NotificationPreference als Routing-Konfiguration, stumm/disabled respektiert
B-NOTIF-MIG: Migration 0120 — Notifications → CommMessages migriert, notifications_legacy View
B-NOTIF-DEPREC: Notification-Routes als deprecated markiert, keine Routes entfernt
B-NOTIF-TEST: 19 Tests in test_notification_migration.py — alle grün
- System-Channel, post_system_message, create_notification Wrapper, Preferences, Unread-Badge, Migration Mapping, Sensitive Fields

PROGRESS.md

@@ -148,13 +148,13 @@
 
 | Task | Status | Forgejo Issue | Verifiziert |
 |------|-------|---------------|------------|
-| B-NOTIF-SYS | `not_started` | — | — |
-| B-NOTIF-EVT | `not_started` | — | — |
+| B-NOTIF-SYS | `done` | — | 19 tests pass |
+| B-NOTIF-EVT | `done` | — | post_system_message + create_notification wrapper |
 | B-NOTIF-UI | `not_started` | — | — |
-| B-NOTIF-PREF | `not_started` | — | — |
-| B-NOTIF-MIG | `not_started` | — | — |
-| B-NOTIF-DEPREC | `not_started` | — | — |
-| B-NOTIF-TEST | `not_started` | — | — |
+| B-NOTIF-PREF | `done` | — | NotificationPreference routing retained |
+| B-NOTIF-MIG | `done` | — | Alembic 0120 migration |
+| B-NOTIF-DEPREC | `done` | — | Routes + create_notification deprecated |
+| B-NOTIF-TEST | `done` | — | tests/test_notification_migration.py 19/19 pass |
 
 ---