feat(L4-L5): KI-Steuerung + XRechnung-Format-Layer (EN16931/CII)

L5 Format-Layer (User-Klaerung: Verkaufsmodul spaeter, Format JETZT):
- einvoice.py: EN16931/XRechnung CII-XML-Generator (ElementTree, XML-Escaping gratis), Pflichtfeld-Validierung mit BT/BG-Codes, Decimal-kommerzielles Rounding, Header-Tax-Breakdown pro VAT-Satz, Profile en16931|xrechnung (XRechnung 3.0)
- POST /einvoice/render (inline->XML), /einvoice/validate (422 mit BT-Fehlliste), /einvoice/render-for (Contract-Resolver einvoice_data() — Andockpunkt Verkaufsmodul, 404 no_data_source ohne Beitrag)

L4 KI-Steuerung:
- POST /documents/suggest: Natuerliche Sprache -> Block-Komposition via zentralem llm_complete (Cost-Tracking, Tenant-Budget), Registry-Sanitizing (ungueltige KI-Bloecke gefiltert, IDs serverseitig), Code-Fence-Stripping, 502 ai_unavailable/invalid_ai_response
- Frontend: KI-Vorschlag-Panel im PrintTemplateEditor (Sparkles-Icon, Prompt-Textarea, Bloecke werden angehaengt), i18n de/en

TDD: Rot 25 failed -> Gruen 25/25 (Validierung, XML-Struktur/Escaping/Summen, Contract-Mocks, API 200/422/403/404, Suggest Mock-LLM/Fence/502). tsc exit 0, Build OK, ruff clean. Doku: api-documentation.md, plugin-development-guide.md (einvoice_data-Contract), PROGRESS.md

frontend/src/api/documents.ts

@@ -294,3 +294,25 @@ export async function renderPrintTemplate(input: RenderInput): Promise<Blob> {
 export function useRenderPrintTemplate() {
   return useMutation({ mutationFn: renderPrintTemplate });
 }
+
+// ─── AI Block Suggestion (L4) ───────────────────────────────────────────────
+
+export interface SuggestInput {
+  prompt: string;
+  entityType?: string | null;
+}
+
+export interface SuggestResult {
+  blocks: DocBlock[];
+  notes: string;
+}
+
+export function useSuggestDocumentBlocks() {
+  return useMutation<SuggestResult, Error, SuggestInput>({
+    mutationFn: (input: SuggestInput) =>
+      apiPost<SuggestResult>('/reports/documents/suggest', {
+        prompt: input.prompt,
+        entity_type: input.entityType ?? null,
+      }),
+  });
+}

frontend/src/components/documents/PrintTemplateEditor.tsx

@@ -7,6 +7,7 @@
 
 import React, { useEffect, useMemo, useState } from 'react';
 import { useTranslation } from 'react-i18next';
+import { Sparkles } from 'lucide-react';
 import { Modal } from '@/components/ui/Modal';
 import { Button } from '@/components/ui/Button';
 import { Input } from '@/components/ui/Input';
@@ -15,6 +16,7 @@ import {
   useCreatePrintTemplate,
   useUpdatePrintTemplate,
   useDocumentPlaceholders,
+  useSuggestDocumentBlocks,
   type DocBlock,
   type Letterhead,
   type PrintTemplate,
@@ -36,6 +38,27 @@ export function PrintTemplateEditor({ template, letterheads, onClose }: PrintTem
   const [letterheadId, setLetterheadId] = useState<string>(template?.letterhead_id ?? '');
   const [entityType, setEntityType] = useState<string>(template?.entity_type ?? 'contact');
   const [blocks, setBlocks] = useState<DocBlock[]>(template?.blocks ?? []);
+  const [aiPrompt, setAiPrompt] = useState('');
+  const [aiNotes, setAiNotes] = useState('');
+  const suggest = useSuggestDocumentBlocks();
+
+  const handleSuggest = async () => {
+    if (!aiPrompt.trim()) return;
+    setAiNotes('');
+    try {
+      const result = await suggest.mutateAsync({ prompt: aiPrompt.trim(), entityType });
+      if (result.blocks.length > 0) {
+        setBlocks((prev) => [...prev, ...result.blocks]);
+        setAiNotes(result.notes);
+        setAiPrompt('');
+        toast.success(t('documents.ai.suggested', '{{count}} Blöcke übernommen', { count: result.blocks.length }));
+      } else {
+        toast.error(t('documents.ai.noBlocks', 'KI hat keine gültigen Blöcke vorgeschlagen'));
+      }
+    } catch {
+      toast.error(t('documents.ai.failed', 'KI-Vorschlag fehlgeschlagen'));
+    }
+  };
 
   const createMutation = useCreatePrintTemplate();
   const updateMutation = useUpdatePrintTemplate();
@@ -140,6 +163,39 @@ export function PrintTemplateEditor({ template, letterheads, onClose }: PrintTem
           </p>
         </div>
 
+        {/* AI suggestion (L4) */}
+        <div className="border border-primary-200 rounded-lg p-3 bg-primary-50/50" data-testid="ai-suggest-panel">
+          <label className="text-xs font-semibold text-primary-700 uppercase flex items-center gap-1" htmlFor="ai-prompt">
+            <Sparkles className="w-3.5 h-3.5" aria-hidden="true" />
+            {t('documents.ai.title', 'KI-Vorschlag')}
+          </label>
+          <p className="text-xs text-secondary-500 mt-1">
+            {t('documents.ai.hint', 'Beschreibe die Vorlage — die KI schlägt Blöcke vor und fügt sie hinzu.')}
+          </p>
+          <div className="flex gap-2 mt-2">
+            <textarea
+              id="ai-prompt"
+              className="flex-1 rounded-md border border-secondary-300 px-3 py-2 text-sm min-h-[60px] focus:outline-none focus:ring-2 focus:ring-primary-500"
+              value={aiPrompt}
+              onChange={(e) => setAiPrompt(e.target.value)}
+              placeholder={t('documents.ai.placeholder', 'z.B. Angebotsvorlage mit Anrede, Firmenname, Leistungsliste und Grußformel')}
+              data-testid="ai-prompt-input"
+            />
+            <Button
+              size="sm"
+              onClick={handleSuggest}
+              isLoading={suggest.isPending}
+              icon={<Sparkles className="w-4 h-4" aria-hidden="true" />}
+              data-testid="ai-suggest-button"
+            >
+              {t('documents.ai.suggest', 'Vorschlagen')}
+            </Button>
+          </div>
+          {aiNotes && (
+            <p className="text-xs text-secondary-600 mt-2" data-testid="ai-suggest-notes">{aiNotes}</p>
+          )}
+        </div>
+
         {/* Block editor (content blocks + letterhead frame preview) */}
         <BlockEditor
           blocks={blocks}

frontend/src/i18n/locales/de.json

@@ -1559,6 +1559,15 @@
       "generated": "Dokument wurde erstellt",
       "generateFailed": "Dokument konnte nicht erstellt werden",
       "noTemplates": "Keine Druckvorlagen für diesen Datentyp. Vorlagen unter Einstellungen → Dokumente anlegen."
+    },
+    "ai": {
+      "title": "KI-Vorschlag",
+      "hint": "Beschreibe die Vorlage — die KI schlägt Blöcke vor und fügt sie hinzu.",
+      "placeholder": "z.B. Angebotsvorlage mit Anrede, Firmenname, Leistungsliste und Grußformel",
+      "suggest": "Vorschlagen",
+      "suggested": "{{count}} Blöcke übernommen",
+      "noBlocks": "KI hat keine gültigen Blöcke vorgeschlagen",
+      "failed": "KI-Vorschlag fehlgeschlagen"
     }
   }
 }

frontend/src/i18n/locales/en.json

@@ -1559,6 +1559,15 @@
       "generated": "Document created",
       "generateFailed": "Document could not be created",
       "noTemplates": "No print templates for this data type. Create templates under Settings → Documents."
+    },
+    "ai": {
+      "title": "AI Suggestion",
+      "hint": "Describe the template — the AI suggests blocks and adds them.",
+      "placeholder": "e.g. quote template with salutation, company name, service list and closing",
+      "suggest": "Suggest",
+      "suggested": "{{count}} blocks added",
+      "noBlocks": "AI did not suggest valid blocks",
+      "failed": "AI suggestion failed"
     }
   }
 }