Subdomain-only gateway routing; drop path prefixes

Collapse the two proxy shapes (path_prefix + host) to a single checkbox: a
service is either port-only, or exposed at <service-name>.<gateway.domain>. Path
routes and their prefix-stripping foot-guns are gone; the subdomain is always the
service name (rename the service to change it).

- Schema: CaddySpec is just `enable` (presence = expose). service_proxy_targets
  returns (expose, port, base_url); Deployment carries `subdomain` (was
  proxy_path/proxy_host). Registry/deploy/mesh/CLI updated in lockstep.
- Generator: compute_routes emits one host route per exposed service/frontend
  (address = name). acme mode = one *.<domain> site with a reverse_proxy matcher
  per service + a file_server matcher per frontend; the :<port> site redirects to
  the dashboard. off mode = a HTTP control plane on :<port> (dashboard at / +
  /api → castle-api); other services are port-only.
- Dashboard/API: castle-app and castle-api are ordinary subdomains. The dashboard
  derives the API base at runtime (castle-api.<domain> on a subdomain, else /api)
  and calls it cross-origin — castle-api already allows CORS *. Frontends build
  with VITE_BASE=/ (served at their subdomain root).
- CLI: `service create` drops --path/--host; --no-proxy makes it port-only.
- Docs/tests reworked for the model; docs use generic placeholders only (no
  personal host/domain/IP details).

castle-api/src/castle_api/nodes.py

@@ -53,7 +53,7 @@ def _deployed_to_summaries(registry: object, hostname: str) -> list[DeploymentSu
                 runner=d.runner,
                 port=d.port,
                 health_path=d.health_path,
-                proxy_path=d.proxy_path,
+                subdomain=d.subdomain,
                 managed=d.managed,
                 schedule=d.schedule,
                 node=hostname,