fix: post-Phase-0 corrections — data-dir bind mounts, reserved subdomains, list_active()

Three related fixes discovered during review of Phase 0 and Phase 1 manifests:

1. validate_rendered_compose(): add allowed_data_dir param. After ${PIC_DATA_DIR}
   substitution, compose templates produce absolute paths; without this the
   validator would reject every service install.  ServiceComposer.write_compose()
   now passes its resolved data_dir so only the designated data directory is
   exempt — /etc, /proc, docker.sock etc. still blocked.

2. _RESERVED_SUBDOMAINS: remove service-level subdomains (mail, calendar, files,
   webdav, webmail). The reserved list should protect PIC infrastructure endpoints
   (api, webui, admin) — not service subdomains that official store services
   (calendar, files, webmail) must be allowed to claim.  Aligns with the
   existing _RESERVED_SUBS in service_registry.py.

3. ServiceRegistry.list_active(): new method returning only installed store
   services (no builtins). This is the forward-looking API that Phase 2 will
   make the primary read path once builtins are deleted. Adding it now unblocks
   the QA agent's test_optional_services_feature.py which was already testing
   the expected Phase 2 behaviour.

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

api/service_composer.py

@@ -156,7 +156,11 @@ class ServiceComposer:
         content = self.render_template(service_id, manifest, template_content)
 
         # Validate before any file I/O so a bad template never touches disk.
-        ok, errs = validate_rendered_compose(content)
+        # Pass the resolved data_dir so that bind mounts created by ${PIC_DATA_DIR}
+        # substitution are allowed; all other absolute paths are still rejected.
+        ok, errs = validate_rendered_compose(
+            content, allowed_data_dir=str(Path(self.data_dir).resolve())
+        )
         if not ok:
             raise ValueError(
                 f'Compose template failed security validation: {"; ".join(errs)}'