feat: add Steuertags concept to separate workflow from content tags

- New steuertag_ids setting to mark tags as workflow-only (not editable)
- DocumentEditModal shows only content tags (non-Steuertags) as editable chips
- Backend preserves Steuertags when saving document tag changes
- ManuellBearbeitenPage renders content tag chips under document title
- New Steuertags settings tab with multi-select and color preview

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

paperless-backend/src/settings/settings.controller.ts

@@ -374,6 +374,36 @@ export class SettingsController {
     return this.settingRepo.findOneByOrFail({ ID: parseInt(id, 10) });
   }
 
+  // === Steuertags ===
+  @Get('steuertags')
+  async getSteuertags() {
+    const setting = await this.settingRepo.findOneBy({ Tag: 'steuertag_ids' });
+    const ids = (setting?.Wert ?? '')
+      .split(',')
+      .map((s) => parseInt(s.trim(), 10))
+      .filter((n) => !isNaN(n));
+    return { ids };
+  }
+
+  @Put('steuertags')
+  async updateSteuertags(@Body() body: { ids: number[] }) {
+    const ids = (body.ids ?? [])
+      .map((id) => Number(id))
+      .filter((n) => !isNaN(n));
+    let setting = await this.settingRepo.findOneBy({ Tag: 'steuertag_ids' });
+    if (!setting) {
+      setting = this.settingRepo.create({
+        Typ: 0,
+        Tag: 'steuertag_ids',
+        Wert: ids.join(','),
+      });
+    } else {
+      setting.Wert = ids.join(',');
+    }
+    await this.settingRepo.save(setting);
+    return { ids };
+  }
+
   // === Korrespondenten ===
   @Get('correspondents')
   async getCorrespondents(