feat(07-04): DkvModule + registry seed + AppModule registration + i18n keys

- dkv.seed.ts: seedDkvModule with slug=dkv-fleet, category=fleet, isSystem=true
- dkv.module.ts: imports ModuleRegistryModule + CalendarModule (CalendarCryptoService)
  provides all 7 DKV services + 2 providers + controller; OnModuleInit seeds registry
- app.module.ts: DkvModule added to imports (ScheduleModule + SettingsModule from Plans 01/03)
- de.json + en.json: complete dkvFleet namespace (50+ keys incl. status, col, form, errors)
- settings namespace extended with categoryGeneral, categorySmtp, smtp sub-object

apps/api/src/dkv/dkv.seed.ts

@@ -0,0 +1,26 @@
+import { ModuleRegistryService } from '../module-registry/module-registry.service';
+
+/**
+ * Seeds the DKV Fleet module into the module registry.
+ *
+ * Called during DkvModule initialization (OnModuleInit) to ensure
+ * the 'dkv-fleet' module record exists in the database.
+ *
+ * Category 'fleet' identifies this as a fleet-management module.
+ * isSystem: true — not tenant-licensable in v1; available to all tenants.
+ */
+export async function seedDkvModule(
+  moduleRegistryService: ModuleRegistryService,
+): Promise<void> {
+  await moduleRegistryService.seedModule({
+    slug: 'dkv-fleet',
+    name: 'DKV Flotte',
+    version: '1.0.0',
+    category: 'fleet',
+    description: {
+      de: 'DKV-Tankkartenrechnungen automatisch verarbeiten',
+      en: 'Automatically process DKV fuel card invoices',
+    },
+    isSystem: true,
+  });
+}