feat: document templates with auto-fill from case data (P1)

- Database: kanzlai.document_templates table with RLS policies
- Seed: 4 system templates (Klageerwiderung UPC, Berufungsschrift,
  Mandatsbestätigung, Kostenrechnung)
- Backend: TemplateService (CRUD + render), TemplateHandler with
  endpoints: GET/POST /api/templates, GET/PUT/DELETE /api/templates/{id},
  POST /api/templates/{id}/render?case_id=X
- Template variables: case.*, party.*, tenant.*, user.*, date.*, deadline.*
- Frontend: /vorlagen page with category filters, template detail/editor,
  render flow (select case -> preview -> copy/download), variable toolbar
- Quick action: "Schriftsatz erstellen" button on case detail page
- Also: resolved merge conflicts between audit-trail and role-based branches,
  added missing Notification/AuditLog types to frontend

frontend/src/app/(app)/vorlagen/neu/page.tsx

@@ -0,0 +1,46 @@
+"use client";
+
+import { useMutation, useQueryClient } from "@tanstack/react-query";
+import { useRouter } from "next/navigation";
+import { api } from "@/lib/api";
+import type { DocumentTemplate } from "@/lib/types";
+import { Breadcrumb } from "@/components/layout/Breadcrumb";
+import { TemplateEditor } from "@/components/templates/TemplateEditor";
+import { toast } from "sonner";
+
+export default function NeueVorlagePage() {
+  const router = useRouter();
+  const queryClient = useQueryClient();
+
+  const createMutation = useMutation({
+    mutationFn: (data: Partial<DocumentTemplate>) =>
+      api.post<DocumentTemplate>("/templates", data),
+    onSuccess: (result) => {
+      queryClient.invalidateQueries({ queryKey: ["templates"] });
+      toast.success("Vorlage erstellt");
+      router.push(`/vorlagen/${result.id}`);
+    },
+    onError: () => toast.error("Fehler beim Erstellen"),
+  });
+
+  return (
+    <div className="animate-fade-in space-y-4">
+      <Breadcrumb
+        items={[
+          { label: "Dashboard", href: "/dashboard" },
+          { label: "Vorlagen", href: "/vorlagen" },
+          { label: "Neue Vorlage" },
+        ]}
+      />
+
+      <h1 className="text-lg font-semibold text-neutral-900">
+        Neue Vorlage erstellen
+      </h1>
+
+      <TemplateEditor
+        onSave={(data) => createMutation.mutate(data)}
+        isSaving={createMutation.isPending}
+      />
+    </div>
+  );
+}