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)

app/plugins/registry.py

@@ -671,6 +671,37 @@ class PluginRegistry:
 
         return plugins_list
 
+    # ── Active UI Manifests (Phase 3) ──
+
+    async def get_active_manifests(self, db: AsyncSession) -> list[dict[str, Any]]:
+        """Return UI manifests for all active plugins.
+
+        Each entry contains the plugin name and its frontend UI contributions
+        (menu_items, page_routes, detail_tabs, settings_pages, dashboard_widgets).
+        """
+        result = await db.execute(select(PluginModel).where(PluginModel.active.is_(True)))
+        active_records = {row.name: row for row in result.scalars().all()}
+
+        manifests: list[dict[str, Any]] = []
+        for name, plugin in self._plugins.items():
+            if name not in active_records:
+                continue
+            m = plugin.manifest
+            manifests.append(
+                {
+                    "name": m.name,
+                    "display_name": m.display_name,
+                    "version": m.version,
+                    "is_core": m.is_core,
+                    "menu_items": [item.model_dump() for item in m.menu_items],
+                    "page_routes": [route.model_dump() for route in m.page_routes],
+                    "detail_tabs": [tab.model_dump() for tab in m.detail_tabs],
+                    "settings_pages": [page.model_dump() for page in m.settings_pages],
+                    "dashboard_widgets": [widget.model_dump() for widget in m.dashboard_widgets],
+                }
+            )
+        return manifests
+
     # ── Internal Helpers ──
 
     async def _get_plugin_record(self, db: AsyncSession, name: str) -> PluginModel | None: