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/report_generator/models.py

@@ -4,7 +4,8 @@ from __future__ import annotations
 
 import uuid
 
-from sqlalchemy import ForeignKey, Index, String, Text
+from sqlalchemy import Boolean, ForeignKey, Index, Integer, String, Text
+from sqlalchemy.dialects.postgresql import JSONB
 from sqlalchemy.dialects.postgresql import UUID as PGUUID
 from sqlalchemy.orm import Mapped, mapped_column
 
@@ -63,3 +64,87 @@ class ReportInstance(Base, TenantMixin, OwnedMixin):
     )
     error_message: Mapped[str | None] = mapped_column(Text, nullable=True)
     created_by: Mapped[uuid.UUID] = mapped_column(PGUUID(as_uuid=True), nullable=False)
+
+
+class Letterhead(Base, TenantMixin, OwnedMixin):
+    """Letterhead (Briefpapier) — page setup + header/footer block composition.
+
+    Phase L1: per-tenant letterhead. ``config`` stores the page geometry
+    (size/orientation/margins) plus header/footer/watermark block lists.
+    Blocks use the same ``{id, type, config}`` shape as print templates so
+    the drag/drop editor can edit both with one component set.
+    """
+
+    __tablename__ = "letterheads"
+    __table_args__ = (
+        Index("ix_letterheads_tenant", "tenant_id"),
+        Index("ix_letterheads_name", "name"),
+    )
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        PGUUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    name: Mapped[str] = mapped_column(String(255), nullable=False)
+    description: Mapped[str] = mapped_column(Text, nullable=False, default="")
+    config: Mapped[dict] = mapped_column(JSONB, nullable=False, default=dict)
+    is_default: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
+    created_by: Mapped[uuid.UUID] = mapped_column(PGUUID(as_uuid=True), nullable=False)
+
+
+class PrintTemplate(Base, TenantMixin, OwnedMixin):
+    """Print template — drag/drop block composition bound to a letterhead.
+
+    Phase L1: ``blocks`` is an ordered JSONB array of
+    ``{id, type, config}`` entries validated against the document block
+    registry. ``entity_type`` selects the module placeholder contribution
+    (e.g. "company" → contacts placeholders).
+    """
+
+    __tablename__ = "print_templates"
+    __table_args__ = (
+        Index("ix_print_templates_tenant", "tenant_id"),
+        Index("ix_print_templates_name", "name"),
+    )
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        PGUUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    name: Mapped[str] = mapped_column(String(255), nullable=False)
+    description: Mapped[str] = mapped_column(Text, nullable=False, default="")
+    letterhead_id: Mapped[uuid.UUID | None] = mapped_column(
+        PGUUID(as_uuid=True),
+        ForeignKey("letterheads.id", ondelete="SET NULL"),
+        nullable=True,
+    )
+    entity_type: Mapped[str] = mapped_column(String(100), nullable=False, default="contact")
+    blocks: Mapped[list] = mapped_column(JSONB, nullable=False, default=list)
+    output_format: Mapped[str] = mapped_column(String(20), nullable=False, default="pdf")
+    created_by: Mapped[uuid.UUID] = mapped_column(PGUUID(as_uuid=True), nullable=False)
+
+
+class DocumentAsset(Base, TenantMixin, OwnedMixin):
+    """Image asset for letterheads/print templates (logos, pictures).
+
+    Stored via the central storage backend; ``data_url`` is rendered into
+    PDFs inline (WeasyPrint URL fetcher allows data: URIs only).
+    """
+
+    __tablename__ = "document_assets"
+    __table_args__ = (
+        Index("ix_document_assets_tenant", "tenant_id"),
+        Index("ix_document_assets_letterhead", "letterhead_id"),
+    )
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        PGUUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    letterhead_id: Mapped[uuid.UUID | None] = mapped_column(
+        PGUUID(as_uuid=True),
+        ForeignKey("letterheads.id", ondelete="CASCADE"),
+        nullable=True,
+    )
+    filename: Mapped[str] = mapped_column(String(255), nullable=False)
+    mime_type: Mapped[str] = mapped_column(String(100), nullable=False)
+    size_bytes: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+    storage_path: Mapped[str] = mapped_column(String(1024), nullable=False)
+    created_by: Mapped[uuid.UUID] = mapped_column(PGUUID(as_uuid=True), nullable=False)