feat(L1-L3): Dokumente-Generator — Briefpapier+Block-System+Drag&Drop-Editor+Renderer

- Briefpapier (letterheads): Seiten-Setup (A4/A5/Letter, Ränder), Header/Footer-Blöcke, Wasserzeichen, Logo-Upload (DocumentAsset, data:-URI-only)
- Druckvorlagen (print_templates): Block-Komposition mit Briefpapier-Ref + entity_type
- Block-Registry (document_blocks.py): text/image/shape(line/rect/circle)/table/spacer/divider/placeholder/pagebreak + Modul-Beiträge via document_blocks()-Contract
- Renderer (document_renderer.py): Blocks→HTML→PDF via WeasyPrint (SSRF-Sandbox data:-URI-only), @page-Frame mit running header/footer, Placeholder-Beispiel-Defaults gegen StrictUndefined
- Contract-Beitrag contacts: document_placeholders/document_data (#359-Muster wie importexport_entities)
- 13 neue Endpoints in documents.py: Letterhead-CRUD, Template-CRUD, Assets, document-blocks, document-placeholders, preview (HTML), render (PDF)
- Migration: Plugin-SQL 0003 (idempotent) + Alembic 0143 (Dual-Path, RLS fail-closed crm_api)
- Frontend: api/documents.ts, Settings→Dokumente (settings_pages), BlockEditor (@dnd-kit Palette/Canvas/Config/Live-Preview-iframe), LetterheadEditor, PrintTemplateEditor, DocumentGenerationDialog (global, ContactDetailPage-Integration)
- i18n de/en, api-documentation.md, plugin-development-guide.md, PROGRESS.md

Verifikation: 32/32 neue Tests + 9/9 Regressionen, tsc exit 0, Build OK 2.79s, Alembic-Fresh-DB 0143 mit RLS bewiesen, ruff clean

frontend/src/routes/index.tsx

@@ -52,6 +52,7 @@ const AutomationDashboardPage = React.lazy(() => import('@/pages/AutomationDashb
 const AgentDashboardPage = React.lazy(() => import('@/pages/AgentDashboard').then(m => ({ default: m.AgentDashboardPage })));
 const AutomationSettingsPage = React.lazy(() => import('@/pages/AutomationSettings').then(m => ({ default: m.AutomationSettingsPage })));
 const ReportsPage = React.lazy(() => import('@/pages/Reports').then(m => ({ default: m.ReportsPage })));
+const DocumentSettingsPage = React.lazy(() => import('@/pages/DocumentSettings').then(m => ({ default: m.DocumentSettingsPage })));
 const TasksPage = React.lazy(() => import('@/pages/Tasks').then(m => ({ default: m.TasksPage })));
 const CommunicationPage = React.lazy(() => import('@/pages/Communication').then(m => ({ default: m.CommunicationPage })));
 const WorkflowsPage = React.lazy(() => import('@/pages/Workflows').then(m => ({ default: m.WorkflowsPage })));
@@ -223,6 +224,7 @@ const router = createBrowserRouter([
           { path: 'custom-fields', element: withSuspense(<CustomFieldsPage />) },
           { path: 'webhooks', element: withSuspense(<SettingsWebhooksPage />) },
           { path: 'workspaces', element: withSuspense(<WorkspaceManagerPage />) },
+          { path: 'documents', element: withSuspense(<DocumentSettingsPage />) },
           { path: 'backup', element: withSuspense(<SettingsBackupPage />) },
           { path: 'rechte', element: <PermissionRoute permission="settings:read">{withSuspense(<SettingsRechtePage />)}</PermissionRoute> },
           { path: '*', element: <ErrorBoundary>{<PluginRouteRenderer />}</ErrorBoundary> },