Correct the models against what a real server actually sends

Every one of these was wrong in a way the mock tests could not see, because
the mocks were written from the same upstream source the models were.

- `SystemLog.created_at`/`updated_at` are integer timestamps, not the date
  strings the OpenAPI spec declares.
- `frequency_id` arrives as a numeric string on recurring invoices and
  expenses, so both now decode through `int_or_numeric_string`.
- `Location.country_id`, `PurchaseOrder.status_id` and `Webhook.format` are
  omitted when empty. The server runs `ConvertEmptyStringsToNull`, so an
  unset `String` sent as `""` becomes NULL and then fails a non-null column
  or an `integer` rule — which meant a create from a `Default` model could
  not succeed.

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

src/models/recurring_invoice.rs

@@ -25,8 +25,9 @@ pub struct RecurringInvoice {
     #[serde(default, deserialize_with = "super::common::null_to_default")]
     pub status_id: String,
     /// How often the invoice recurs. Invoice Ninja returns this as an integer
-    /// enum (1 = daily, 2 = weekly, … ), not a human-readable string.
-    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    /// enum (1 = daily, 2 = weekly, … ), not a human-readable string. A real
+    /// server sends it as a numeric string; both decode.
+    #[serde(default, deserialize_with = "super::common::int_or_numeric_string")]
     pub frequency_id: i64,
     /// Number of cycles left to send; `-1` means it recurs indefinitely.
     #[serde(default, deserialize_with = "super::common::null_to_default")]