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/schemas.py

@@ -72,3 +72,93 @@ class ReportResponse(BaseModel):
     created_by: str
     created_at: datetime | None = None
     updated_at: datetime | None = None
+
+
+# ─── Documents Generator (Phase L1-L3) ──────────────────────────────────────
+
+
+class LetterheadCreate(BaseModel):
+    name: str = Field(..., min_length=1, max_length=255)
+    description: str = Field("", max_length=2000)
+    config: dict = Field(default_factory=dict)
+    is_default: bool = False
+
+
+class LetterheadUpdate(BaseModel):
+    name: str | None = Field(None, min_length=1, max_length=255)
+    description: str | None = Field(None, max_length=2000)
+    config: dict | None = None
+    is_default: bool | None = None
+
+
+class LetterheadResponse(BaseModel):
+    id: str
+    name: str
+    description: str
+    config: dict
+    is_default: bool
+    created_by: str
+    created_at: datetime | None = None
+    updated_at: datetime | None = None
+
+
+class PrintTemplateCreate(BaseModel):
+    name: str = Field(..., min_length=1, max_length=255)
+    description: str = Field("", max_length=2000)
+    letterhead_id: str | None = None
+    entity_type: str = Field("contact", max_length=100)
+    blocks: list[dict] = Field(default_factory=list)
+    output_format: str = Field("pdf", pattern="^(pdf|print)$")
+
+
+class PrintTemplateUpdate(BaseModel):
+    name: str | None = Field(None, min_length=1, max_length=255)
+    description: str | None = Field(None, max_length=2000)
+    letterhead_id: str | None = None
+    entity_type: str | None = Field(None, max_length=100)
+    blocks: list[dict] | None = None
+    output_format: str | None = Field(None, pattern="^(pdf|print)$")
+
+
+class PrintTemplateResponse(BaseModel):
+    id: str
+    name: str
+    description: str
+    letterhead_id: str | None = None
+    entity_type: str
+    blocks: list[dict]
+    output_format: str
+    created_by: str
+    created_at: datetime | None = None
+    updated_at: datetime | None = None
+
+
+class DocumentPreviewRequest(BaseModel):
+    """Preview: render blocks to HTML (live preview in the editor)."""
+
+    blocks: list[dict]
+    letterhead_config: dict | None = None
+    entity_type: str | None = None
+    data: dict | None = None
+
+
+class DocumentPreviewResponse(BaseModel):
+    html: str
+
+
+class DocumentRenderRequest(BaseModel):
+    """Render a stored print template with entity data to PDF."""
+
+    entity_type: str
+    entity_id: str
+    output_format: str = Field("pdf", pattern="^(pdf|print)$")
+
+
+class DocumentAssetResponse(BaseModel):
+    id: str
+    letterhead_id: str | None = None
+    filename: str
+    mime_type: str
+    size_bytes: int
+    data_url: str | None = None
+    created_at: datetime | None = None