fix(07): CR-01 rename import return field imported→count to match frontend

Backend dkv.service.ts returned { imported } but frontend read result.count,
causing the success toast to always display "undefined Fahrzeuge importiert".
Align backend field name to count and update the dkv-api.ts return type to
include mode for completeness.

apps/api/src/dkv/dkv.service.ts

@@ -498,7 +498,7 @@ export class DkvService {
     }
 
     this.logger.log(`DKV CSV import: ${vehicles.length} vehicles, mode=${mode}, tenant=${tenantId}`);
-    return { imported: vehicles.length, mode };
+    return { count: vehicles.length, mode };
   }
 
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