fix(07): WR-01 close nodemailer transport in finally to prevent pool leak

Each sendExportEmail call created a new nodemailer transport which was never
closed, leaving the internal SMTP connection pool alive. With 3-retry backoff,
up to 3 leaked transports per invoice accumulate over time and can exhaust OS
socket limits. Add transport.close() in a finally block.

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

@@ -103,6 +103,11 @@ export class DkvMailService {
       );
       // RETHROW — caller (DkvService) handles exponential backoff retries (D-16)
       throw error;
+    } finally {
+      // Always close the transport to release the SMTP connection pool (WR-01).
+      // Without this, repeated sends (especially with retry backoff) accumulate
+      // open connections and can exhaust OS socket limits.
+      transport.close();
     }
   }
 }