fix: fall back to cell effective domain when email service domain not configured

When the email store service is installed but no explicit domain has been
set in its config, _provision_email now falls back to
config_manager.get_effective_domain() so peer account creation works
immediately without requiring a separate config step.

Also threads config_manager into AccountManager.__init__ (optional kwarg,
no existing callers break) so the fallback is available without a global
import.

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

api/managers.py

@@ -80,6 +80,7 @@ service_composer = ServiceComposer(config_manager=config_manager, data_dir=DATA_
 account_manager  = AccountManager(
     service_registry=service_registry,
     data_dir=DATA_DIR,
+    config_manager=config_manager,
     email_manager=email_manager,
     calendar_manager=calendar_manager,
     file_manager=file_manager,