feat: replace hardcoded service names with ServiceRegistry-driven Caddy and CoreDNS config

Previously, CaddyManager and NetworkManager contained hardcoded lists of
service names (calendar, files, mail, webdav, etc.), meaning every new
service required a code change to appear in Caddy routes and DNS records.
Now both managers accept a service_registry parameter and derive their
service lists dynamically from the registry at runtime.

- CaddyManager: new _build_registry_service_routes() and
  _http01_service_pairs() methods pull routes from the registry
- NetworkManager: new _get_service_subdomains() method returns registry
  subdomains with a hardcoded fallback when no registry is wired in;
  _build_dns_records, stale-record detection, and service name sets all
  use the registry
- managers.py: service_registry constructed before network_manager so it
  can be injected into both CaddyManager and NetworkManager
- service_registry.py: validation chokepoint in get_caddy_routes() rejects
  invalid subdomain/backend values and reserved service names
- service_store_manager.py: _validate_manifest now validates top-level
  subdomain, backend, extra_subdomains, and extra_backends fields
- tests: 24 new tests covering registry-driven routing and DNS subdomain
  generation (test_caddy_registry_integration.py)

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

api/app.py

@@ -44,6 +44,9 @@ from managers import (
     caddy_manager,
     ddns_manager, service_store_manager,
     connectivity_manager,
+    service_registry,
+    service_composer,
+    account_manager,
     firewall_manager, EventType,
 )
 # Re-exports: tests do `from app import CellManager` and `from app import _resolve_peer_dns`