feat: add create document feature via web interface

- Backend: new POST /api/documents endpoint with DocumentService.createDocument()
  reusing DocumentUpdateDTO; handles file upload, tags, sender, receivers
- Frontend: new /documents/new route with same four-section form as edit page
  (Wer & Wann, Beschreibung, Transkription, Datei) but with empty fields
- Home page: subtle '+ Neues Dokument' link above the document list

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/generated/api.ts

@@ -100,6 +100,22 @@ export interface paths {
         patch?: never;
         trace?: never;
     };
+    "/api/documents": {
+        parameters: {
+            query?: never;
+            header?: never;
+            path?: never;
+            cookie?: never;
+        };
+        get?: never;
+        put?: never;
+        post: operations["createDocument"];
+        delete?: never;
+        options?: never;
+        head?: never;
+        patch?: never;
+        trace?: never;
+    };
     "/api/admin/trigger-import": {
         parameters: {
             query?: never;
@@ -635,6 +651,30 @@ export interface operations {
             };
         };
     };
+    createDocument: {
+        parameters: {
+            query?: never;
+            header?: never;
+            path?: never;
+            cookie?: never;
+        };
+        requestBody?: {
+            content: {
+                "multipart/form-data": components["schemas"]["DocumentUpdateDTO"];
+            };
+        };
+        responses: {
+            /** @description OK */
+            200: {
+                headers: {
+                    [name: string]: unknown;
+                };
+                content: {
+                    "*/*": components["schemas"]["Document"];
+                };
+            };
+        };
+    };
     triggerMassImport: {
         parameters: {
             query?: never;