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/products.json

@@ -0,0 +1,37 @@
+{
+  "data": [
+    {
+      "archived_at": 0,
+      "assigned_user_id": "",
+      "cost": 0,
+      "created_at": 0,
+      "custom_value1": "",
+      "custom_value2": "",
+      "custom_value3": "",
+      "custom_value4": "",
+      "documents": [],
+      "id": "",
+      "in_stock_quantity": 0,
+      "income_account_id": "",
+      "is_deleted": false,
+      "max_quantity": 0,
+      "notes": "",
+      "price": 0,
+      "product_image": "",
+      "product_key": "",
+      "quantity": 0,
+      "stock_notification": false,
+      "stock_notification_threshold": 0,
+      "tags": [],
+      "tax_id": "",
+      "tax_name1": "",
+      "tax_name2": "",
+      "tax_name3": "",
+      "tax_rate1": 0,
+      "tax_rate2": 0,
+      "tax_rate3": 0,
+      "updated_at": 0,
+      "user_id": ""
+    }
+  ]
+}