feat: tag documents as Posteingang when AM entry is missing during upload check

- Add agrarmonitor_tag_posteingang setting (default empty)
- When a document is not found in Agrarmonitor, move it back to Posteingang
  tag instead of skipping (if tagPosteingang is configured)
- Expose tagPosteingang in polling config API and settings UI

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

paperless-frontend/src/api/settings.ts

@@ -202,6 +202,7 @@ export interface AgrarmonitorPollingConfig {
   tagVerbucht: string;
   tagHochgeladen: string;
   linkField: string;
+  tagPosteingang: string;
 }
 
 export interface AgrarmonitorPollingResult {

paperless-frontend/src/pages/SettingsPage.tsx

@@ -2588,6 +2588,9 @@ function AgrarmonitorTab() {
             <Form.Item name="tagHochgeladen" label="Tag-ID: Hochgeladen in Agrarmonitor">
               <Input placeholder="3" style={{ width: 120 }} />
             </Form.Item>
+            <Form.Item name="tagPosteingang" label="Tag-ID: Posteingang (bei fehlendem AM-Beleg)">
+              <Input placeholder="1" style={{ width: 120 }} />
+            </Form.Item>
             <Form.Item name="linkField" label="Custom Field: Agrarmonitor-Link">
               <Select allowClear placeholder="Kein Feld ausgewählt" style={{ width: 280 }}>
                 {customFields.map(f => (