feat(capability): BYO SMTP — per-tenant email server via the credential registry

SmtpProvider is now scope-aware: it resolves the tenant's own SMTP identity from
the IiosProviderCredential store (providerType SMTP) and sends from it, falling
back to the platform env identity when unset (env fallback applies only to the
platform identity, never a tenant's server). Registered whenever env SMTP OR the
credential store is present; fails closed as NOT_CONFIGURED when neither exists.
Credential endpoints accept SMTP with per-type validation. 18 SMTP tests green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/iios-service/src/capability/provider-credential.service.ts

@@ -18,6 +18,9 @@ function hintsFor(providerType: string, config: ProviderConfig): Record<string,
     const sid = String(config.accountSid ?? '');
     return { fromNumber: config.fromNumber ?? null, sidLast4: sid.slice(-4) };
   }
+  if (providerType === 'SMTP') {
+    return { host: config.host ?? null, port: config.port ?? null, user: config.user ?? null, fromEmail: config.fromEmail ?? null, fromName: config.fromName ?? null };
+  }
   return {};
 }