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/expenses.json

@@ -0,0 +1,55 @@
+{
+  "data": [
+    {
+      "amount": 0,
+      "archived_at": 0,
+      "assigned_user_id": "",
+      "bank_id": "",
+      "calculate_tax_by_amount": false,
+      "category_id": "",
+      "client_id": "",
+      "created_at": 0,
+      "currency_id": "",
+      "custom_value1": "",
+      "custom_value2": "",
+      "custom_value3": "",
+      "custom_value4": "",
+      "date": "",
+      "documents": [],
+      "e_invoice": {},
+      "entity_type": "",
+      "exchange_rate": 0,
+      "expense_currency_id": "",
+      "foreign_amount": 0,
+      "id": "",
+      "invoice_currency_id": "",
+      "invoice_documents": false,
+      "invoice_id": "",
+      "is_deleted": false,
+      "number": "",
+      "payment_date": "",
+      "payment_type_id": "",
+      "private_notes": "",
+      "project_id": "",
+      "public_notes": "",
+      "recurring_expense_id": "",
+      "should_be_invoiced": false,
+      "tags": [],
+      "tax_amount1": 0,
+      "tax_amount2": 0,
+      "tax_amount3": 0,
+      "tax_name1": "",
+      "tax_name2": "",
+      "tax_name3": "",
+      "tax_rate1": 0,
+      "tax_rate2": 0,
+      "tax_rate3": 0,
+      "transaction_id": "",
+      "transaction_reference": "",
+      "updated_at": 0,
+      "user_id": "",
+      "uses_inclusive_taxes": false,
+      "vendor_id": ""
+    }
+  ]
+}