feat: Plugin-System Umbau — 6 Phasen komplett abgeschlossen

Phase 1: Contracts konsequent nutzen
- 12 neue contracts.py erstellt (alle 19 Plugins haben jetzt contracts)
- 4 bestehende contracts.py an zentrale ContractRegistry angepasst
- Alle 19 Plugins haben on_deactivate mit Contract-Unregister
- 0 echte problematische INTER-Plugin Imports

Phase 2: Hooks/Filters-System
- app/core/hooks.py (HookRegistry mit actions + filters)
- 15 Hook-Punkte in Core-Services (contact, auth, mail, calendar, user, dms)
- BasePlugin.on_deactivate meldet alle Hooks ab

Phase 3: Plugin-Isolation
- scripts/check_cross_plugin_imports.py (Linting-Regel)
- .github/workflows/check-cross-plugin-imports.yml (CI/CD)
- .pre-commit-cross-plugin.yaml (Pre-commit Hook)
- 155 Dateien geprueft, 0 Verstoesse

Phase 4: Plugin-Versioning
- app/plugins/semver.py (SemVer mit Parse, Compare, Pre-release)
- migration_runner.py erweitert: run_migration_down, rollback_to_version
- manifest.py: min_app_version Feld
- registry.py: App-Version-Compatibility-Check bei Installation
- GET /api/v1/plugins/updates Endpoint

Phase 5: Marketplace-Vorbereitung
- app/plugins/signature.py (Ed25519 Signatur-Validierung)
- app/plugins/quarantine.py (Plugin-Quarantine mit Validierung)
- app/models/plugin_allowlist.py + Migration 0046
- manifest.py: author, license, homepage, icon, screenshots, changelog, marketplace_tags, price
- registry.py: discover_external(), discover_all()
- POST /api/v1/plugins/install-marketplace (deaktiviert)

Phase 6: Manifest-Anpassung
- manifest.py: 12 neue Felder + SemVer/Hook-Name Validierung
- MANIFEST_SCHEMA_DOC aktualisiert
- Alle 19 Plugin-Manifeste aktualisiert
- Frontend PluginUiManifest Typ erweitert

Zusaetzliche Bug-Fixes:
- test_sample-Modul erstellt
- conftest.py Deadlock-Prevention
- SESSION_COOKIE_SECURE=true
- dump.rdb aus Git entfernt + .gitignore
- backup.py datetime.utcnow -> func.now()
- system_settings.py JSONB-Import nach oben
- tax.py Mapped[float] -> Mapped[Decimal]
- notification.py type_key-Laengen vereinheitlicht

Tests: 91 neue Tests, alle bestanden

app/plugins/builtins/calendar/routes.py

@@ -410,6 +410,11 @@ async def create_entry(
             )
 
     assigned_to = _parse_uuid(body.assigned_to, "assigned_to") if body.assigned_to else None
+
+    # ── Hook: calendar.before_appointment (Action) ──
+    from app.core.hooks import do_action
+    await do_action("calendar.before_appointment", body=body, tenant_id=tenant_id, user_id=user_id, cal_id=cal_id)
+
     entry = CalendarEntry(
         tenant_id=tenant_id,
         calendar_id=cal_id,
@@ -432,6 +437,10 @@ async def create_entry(
     db.add(entry)
     await db.flush()
 
+    # ── Hook: calendar.after_appointment (Action) ──
+    from app.core.hooks import do_action
+    await do_action("calendar.after_appointment", entry_id=str(entry.id), tenant_id=tenant_id, user_id=user_id)
+
     # Schedule reminder ARQ job if reminder is set
     if body.reminder:
         try: