fix: valid Caddyfile handle_errors + frontend health-gate in deploys (#20)

The learning-platform (Caddy) container crash-looped since this morning's
89d3395: `Content-Type application/json` is not a valid subdirective of
`respond` — a Caddyfile parse error aborts Caddy at startup, taking the
whole app offline for authenticated users while every CI run stayed green.
Verified with caddy v2.10.0: "unrecognized subdirective 'Content-Type',
at Caddyfile:53"; the fixed file validates clean.

CI was blind to this twice over: test.yml swaps the real Caddyfile for
Caddyfile.test in the test image, and the deploy health-gate from #18
only covers PocketBase.

- Caddyfile: set Content-Type via a `header @api` directive before the
  `respond`; behaviour of the JSON error response is unchanged
- infra/*/site/deploy-playbook.yml: frontend health-gate (docker exec
  wget --spider on the container) with log dump + abort on failure, and
  an always-on post-deploy smoke report (compose ps, proxy health,
  team_members auth-methods, PocketBase log tail) for visibility behind
  the auth perimeter

Closes #20

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Caddyfile

@@ -48,10 +48,13 @@
     handle_errors {
         # Don't mask API errors with the SPA shell — return a proper
         # JSON error so the frontend can display a meaningful message.
+        # NOTE: `respond` only accepts `body`/`close` subdirectives; the
+        # Content-Type must be set via a separate `header` directive. An
+        # invalid subdirective is a Caddyfile parse error and crash-loops
+        # the container at startup (issue #20).
         @api path /api/*
-        respond @api `{"code":{err.status_code},"message":"Backend unavailable"}` {err.status_code} {
-            Content-Type application/json
-        }
+        header @api Content-Type application/json
+        respond @api `{"code":{err.status_code},"message":"Backend unavailable"}` {err.status_code}
 
         rewrite * /index.html
         file_server