Pin the response shapes captured from a real server

Twice now a mock written from the upstream source passed while a real server
disagreed — the system-log timestamps and the `/subscriptions/steps` shape —
and `frequency_id` and several empty-string fields were wrong the same way.
Reading the source harder was not going to find the next one.

`tests/fixtures/*.json` hold the *shapes* of real responses: every key, with
its JSON type, and no values at all. `real_shapes.rs` decodes each one and
checks that a decode plus re-serialise loses no key, so a field the models
silently drop now fails a test. It also refuses a fixture holding any real
value, and refuses a fixture that no test reads.

`seed_sample_data.rs` fills a disposable instance with one row of each kind
and `capture_shapes.rs` captures the shapes back out with GET requests only,
so the set can be refreshed when a divergence turns up.

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

tests/fixtures/health_check.json

@@ -0,0 +1,37 @@
+{
+  "api_version": "",
+  "cache_enabled": false,
+  "env_writable": false,
+  "exchange_rate_api_not_configured": false,
+  "exec": false,
+  "extensions": [
+    {
+      "gd": false
+    }
+  ],
+  "file_permissions": "",
+  "flutter_renderer": "",
+  "is_docker": false,
+  "jobs_pending": 0,
+  "mail_mailer": "",
+  "open_basedir": false,
+  "pdf_engine": "",
+  "pending_migrations": false,
+  "phantom_enabled": false,
+  "php_version": {
+    "current_php_cli_version": "",
+    "current_php_version": "",
+    "is_okay": false,
+    "memory_limit": "",
+    "minimum_php_version": ""
+  },
+  "queue": "",
+  "queue_data": {
+    "failed": 0,
+    "last_error": "",
+    "pending": 0
+  },
+  "simple_db_check": false,
+  "system_health": false,
+  "trailing_slash": false
+}