Phase 3: Plugin-UI-System (WordPress-Style)

Backend:
- PluginManifest um 5 neue UI-Felder erweitert: menu_items, page_routes,
  detail_tabs, settings_pages, dashboard_widgets (FrontendMenuItem,
  FrontendPageRoute, FrontendDetailTab, FrontendSettingsPage,
  FrontendDashboardWidget)
- GET /api/v1/plugins/active-manifests Endpoint liefert UI-Manifeste
  aller aktiven Plugins
- Registry.get_active_manifests() + PluginService.get_active_manifests()
- 12 Built-in Plugins mit UI-Manifest-Daten gefuellt (menu_items,
  page_routes, detail_tabs, settings_pages)
- Plugin-Install-System: POST /upload (ZIP), POST /install-url (URL)
  mit Validierung (Manifest, dangerous imports, SQL migrations)

Frontend:
- pluginStore.ts (Zustand) mit PluginUiManifest Typen + Selektoren
- useActivePluginManifests() React Query Hook
- PluginRegistry.tsx — fetcht Manifeste beim App-Start
- PluginLoader.tsx — dynamisches React.lazy() mit ErrorBoundary
- PluginRouteRenderer.tsx — Catch-all fuer Plugin-Routes
- routes/index.tsx — Catch-all Routes fuer Plugin-Pages + Settings
- Sidebar.tsx — dynamische Plugin Menu-Items mit Grouping + Icons
- Settings.tsx — dynamische Plugin Settings-Pages
- ContactDetail.tsx — dynamische Plugin Detail-Tabs mit Permissions
- AppShell.tsx — PluginRegistry Provider eingebunden
- SettingsPlugins.tsx — Install-UI (ZIP Upload + URL Install)
- plugins.ts — useUploadPlugin() + useInstallPluginFromUrl() Hooks

Docs & Templates:
- docs/plugin-development-guide.md — komplette Entwickler-Doku
- templates/plugin-template/ — Boilerplate mit allen Manifest-Feldern

Tests:
- 34 Vitest-Tests (PluginRegistry, PluginLoader, PluginRouteRenderer,
  pluginStore) — alle bestanden
- TSC: keine neuen Errors (nur pre-existing Dms.tsx)

templates/plugin-template/services.py

@@ -0,0 +1,39 @@
+"""
+Business logic services for the example plugin.
+
+Customize these for your plugin's business logic.
+"""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from sqlalchemy.ext.asyncio import AsyncSession
+
+
+class ExampleService:
+    """Service class for example plugin business logic."""
+
+    def __init__(self, db: AsyncSession):
+        self.db = db
+
+    async def list_items(self, tenant_id: str, skip: int = 0, limit: int = 100) -> dict:
+        """List items for a tenant."""
+        # TODO: Implement with actual database queries
+        return {"items": [], "total": 0}
+
+    async def create_item(self, tenant_id: str, data: dict) -> dict:
+        """Create a new item."""
+        # TODO: Implement with actual database operations
+        return {"id": "new-uuid", **data}
+
+    async def get_item(self, item_id: str, tenant_id: str) -> dict | None:
+        """Get a single item by ID."""
+        # TODO: Implement with actual database queries
+        return None
+
+    async def delete_item(self, item_id: str, tenant_id: str) -> bool:
+        """Delete an item by ID."""
+        # TODO: Implement with actual database operations
+        return True