Flatten proxy config to a bool (drop the caddy key)

CaddySpec had only `enable` left (path_prefix/host removed; extra_snippets was
unused), so `proxy: { caddy: {} }` was three levels of nesting for a checkbox —
and it invited drift (supabase silently reverted to proxy.caddy.host). Replace it
with a plain `ServiceSpec.proxy: bool`.

- Model: remove ProxySpec/CaddySpec; `proxy: bool = False`. Readers simplified to
  `bool(svc.proxy)` (generator, castle-api summaries, CLI info).
- CLI create / dashboard editor + create form: write `proxy: true` / a checkbox.
- Configs migrated to `proxy: true` (and supabase's stale proxy.caddy.host block
  removed).
- Docs (registry, dns-and-tls, design, stack guides) + tests/fixtures updated;
  also fixed lingering path-model staleness in the stack guides.

castle-api/src/castle_api/routes.py

@@ -103,7 +103,7 @@ def _summary_from_service(
     if svc.expose and svc.expose.http:
         port = svc.expose.http.internal.port
         health_path = svc.expose.http.health_path
-    subdomain = name if (svc.proxy and svc.proxy.caddy and svc.proxy.caddy.enable) else None
+    subdomain = name if svc.proxy else None
 
     managed = bool(svc.manage and svc.manage.systemd and svc.manage.systemd.enable)
 
@@ -278,7 +278,7 @@ def _service_from_spec(name: str, svc: ServiceSpec, config: object) -> ServiceSu
         port = svc.expose.http.internal.port
         health_path = svc.expose.http.health_path
     # Exposed at <name>.<domain> when the proxy checkbox is on.
-    subdomain = name if (svc.proxy and svc.proxy.caddy and svc.proxy.caddy.enable) else None
+    subdomain = name if svc.proxy else None
 
     managed = bool(svc.manage and svc.manage.systemd and svc.manage.systemd.enable)
     systemd_info = _make_systemd_info(name) if managed else None