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).

cli/src/castle_cli/main.py

@@ -90,9 +90,11 @@ def _add_service_create(sub: argparse._SubParsersAction, kind: str) -> None:
     if kind == "service":
         p.add_argument("--port", type=int, help="HTTP port")
         p.add_argument("--health", default="/health", help="Health path (default: /health)")
-        p.add_argument("--path", help="Gateway proxy prefix (default: /<name>)")
-        p.add_argument("--host", help="Route by hostname instead of a path prefix")
-        p.add_argument("--no-proxy", action="store_true", help="Don't add a gateway route")
+        p.add_argument(
+            "--no-proxy",
+            action="store_true",
+            help="Port-only; don't expose at <name>.<gateway.domain>",
+        )
     else:
         p.add_argument("--schedule", default="0 2 * * *", help="Cron schedule (default: 0 2 * * *)")