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

app/plugins/builtins/contacts/contracts.py

@@ -308,6 +308,47 @@ class ContactsContract:
         )
         return _serialize_contact(contact)
 
+    # ─── Document Generator contribution (Phase L1, #359 pattern) ───
+    # The documents generator resolves placeholders + entity data via these
+    # contract hooks. Same philosophy as importexport_entities(): the module
+    # owns its domain data, the generic renderer stays module-agnostic.
+
+    @staticmethod
+    def document_entity_types() -> list[str]:
+        """Entity types this plugin serves in the documents generator."""
+        return ["contact", "company", "person"]
+
+    @staticmethod
+    def document_placeholders(entity_type: str) -> list[dict]:
+        """Placeholder descriptors (key/label/example) for the drag/drop editor."""
+        return _placeholders_for(entity_type)
+
+    @staticmethod
+    async def document_data(
+        db: AsyncSession,
+        tenant_id: Any,
+        entity_id: Any,
+        entity_type: str,
+    ) -> dict[str, Any]:
+        """Load one entity as template data ({} when not found)."""
+        contact = (
+            await db.execute(
+                select(Contact).where(
+                    Contact.id == entity_id,
+                    Contact.tenant_id == tenant_id,
+                    Contact.deleted_at.is_(None),
+                )
+            )
+        ).scalar_one_or_none()
+        if contact is None:
+            return {}
+        fields = _contacts_document_fields()
+        data: dict[str, Any] = {}
+        for key in fields:
+            value = getattr(contact, key, None)
+            data[key] = value if value is not None else ""
+        return data
+
     @classmethod
     def get_function(cls, name: str):
         """Return a callable exposed by this contract, or None if absent."""
@@ -318,3 +359,70 @@ class ContactsContract:
 
 _contract = ContactsContract()
 get_contract_registry().register("contacts", _contract)
+
+
+def _contacts_document_fields() -> dict[str, str]:
+    """Contact/company fields available in document templates (L1).
+
+    Keys map to Contact model attributes; labels/examples feed the
+    drag/drop editor palette and the preview fallback values.
+    """
+    return {
+        "displayname": "Anzeigename",
+        "firstname": "Vorname",
+        "surname": "Nachname",
+        "name": "Firmenname",
+        "email": "E-Mail",
+        "email_1": "E-Mail 1",
+        "email_2": "E-Mail 2",
+        "phone": "Telefon",
+        "phone_1": "Telefon 1",
+        "phone_2": "Telefon 2",
+        "mobile": "Mobil",
+        "website": "Website",
+        "industry": "Branche",
+        "city": "Stadt",
+        "postalcode": "PLZ",
+        "country": "Land",
+        "vat_code": "USt-IdNr.",
+        "function": "Funktion",
+        "department": "Abteilung",
+    }
+
+
+_CONTACT_DOC_EXAMPLES = {
+    "displayname": "Max Mustermann",
+    "firstname": "Max",
+    "surname": "Mustermann",
+    "name": "Muster GmbH",
+    "email": "max@example.com",
+    "email_1": "max@example.com",
+    "email_2": "buero@example.com",
+    "phone": "+49 30 123456",
+    "phone_1": "+49 30 123456",
+    "phone_2": "+49 171 1234567",
+    "mobile": "+49 171 1234567",
+    "website": "https://example.com",
+    "industry": "IT",
+    "city": "Berlin",
+    "postalcode": "10115",
+    "country": "Deutschland",
+    "vat_code": "DE123456789",
+    "function": "Geschäftsführer",
+    "department": "Vertrieb",
+}
+
+
+def _placeholders_for(entity_type: str) -> list[dict]:
+    """Placeholder descriptors for contact/company templates."""
+    if entity_type not in ("contact", "company", "person"):
+        return []
+    fields = _contacts_document_fields()
+    result = []
+    for key, label in fields.items():
+        result.append({
+            "key": key,
+            "label": label,
+            "example": _CONTACT_DOC_EXAMPLES.get(key, "…"),
+        })
+    return result