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.

cli/src/castle_cli/commands/info.py

@@ -119,7 +119,7 @@ def run_info(args: argparse.Namespace) -> int:
             print(f"  {BOLD}port{RESET}:        {http.internal.port}")
             if http.health_path:
                 print(f"  {BOLD}health{RESET}:      {http.health_path}")
-        if service.proxy and service.proxy.caddy and service.proxy.caddy.enable:
+        if service.proxy:
             print(f"  {BOLD}subdomain{RESET}:   {name}.<gateway.domain>")
         if service.manage and service.manage.systemd:
             sd = service.manage.systemd