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/pages/Settings.tsx

@@ -5,7 +5,7 @@ import { usePluginStore } from '@/store/pluginStore';
 import { useUIStore } from '@/store/uiStore';
 import { usePermission } from '@/hooks/usePermission';
 import { useAuthStore } from '@/store/authStore';
-import { Settings, Mail, Bell, Sparkles, Bot, Shield, Users, UsersRound, Package, ArrowLeft } from 'lucide-react';
+import { Settings, Mail, Bell, Sparkles, Bot, Shield, Users, UsersRound, Package, ArrowLeft, FileText } from 'lucide-react';
 
 const ICON_MAP: Record<string, React.ComponentType<{ className?: string }>> = {
   Settings,
@@ -16,6 +16,7 @@ const ICON_MAP: Record<string, React.ComponentType<{ className?: string }>> = {
   Shield,
   Users,
   UsersRound,
+  FileText,
 };
 
 const FALLBACK_ICON = Package;