feat: Phase 2 — remove builtins layer, ServiceRegistry is installed-only

Builtins (email/calendar/files) are no longer baked into the API image.
ServiceRegistry now only knows about installed store services. When nothing
is installed, Caddy and DNS get no service routes — no hardcoded fallback.

Changes:
- service_registry.py: remove _BUILTINS_DIR, _builtin_ids, _builtin_manifest,
  _load_manifest; get() and list_all() now delegate entirely to installed services
- caddy_manager.py: remove _build_core_service_routes(); remove hardcoded
  fallback pairs from _http01_service_pairs(); empty registry → api block only
- network_manager.py: _get_service_subdomains() returns [] when no registry
- api/services/builtins/: deleted (email, calendar, files manifests)
- Tests updated throughout: removed builtin-dependent assertions, added
  installed-service fixtures, updated fallback expectations to api-only

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

api/network_manager.py

@@ -268,7 +268,7 @@ class NetworkManager(BaseServiceManager):
                 return subs
             except Exception as exc:
                 logger.warning('_get_service_subdomains: registry error: %s', exc)
-        return ['calendar', 'files', 'mail', 'webmail', 'webdav']
+        return []
 
     def _build_dns_records(self, cell_name: str, ip_range: str) -> List[Dict]:
         """Build the standard set of DNS A records.