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).

core/src/castle_core/manifest.py

@@ -146,12 +146,10 @@ class ExposeSpec(BaseModel):
 
 
 class CaddySpec(BaseModel):
+    # The "expose" checkbox: when present and enabled, the gateway routes
+    # <service-name>.<gateway.domain> to this service (whole host → backend root).
+    # The subdomain is always the service name — rename the service to change it.
     enable: bool = True
-    path_prefix: str | None = None
-    # Route by hostname instead of (or alongside) a path prefix. The whole host
-    # maps to the backend root, so an app built with base="/" works unchanged —
-    # the fix for proxying a root-based SPA the gateway can't rebuild.
-    host: str | None = None
     extra_snippets: list[str] = Field(default_factory=list)