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/location.rs

@@ -18,7 +18,14 @@ pub struct Location {
     pub state: String,
     #[serde(default, deserialize_with = "super::common::null_to_default")]
     pub postal_code: String,
-    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    /// Omitted when empty: the server validates it as an integer, so an unset
+    /// `""` is rejected with a 422 and a default `Location` could not be
+    /// created.
+    #[serde(
+        default,
+        deserialize_with = "super::common::null_to_default",
+        skip_serializing_if = "String::is_empty"
+    )]
     pub country_id: String,
     #[serde(default, deserialize_with = "super::common::null_to_default")]
     pub user_id: String,

src/models/payment.rs

@@ -38,7 +38,9 @@ pub struct Payment {
 
 #[derive(Debug, Clone, Default, Serialize, Deserialize)]
 pub struct PaymentInvoice {
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
     pub invoice_id: String,
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
     pub amount: f64,
 
     #[serde(flatten)]

src/models/purchase_order.rs

@@ -23,7 +23,14 @@ pub struct PurchaseOrder {
     pub po_number: String,
     #[serde(default, deserialize_with = "super::common::null_to_default")]
     pub quote_number: String,
-    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    /// Omitted when empty: the server turns `""` into `NULL` and then fails
+    /// the insert on a non-null column with a 500, so a new purchase order
+    /// must leave it out and take the server's default.
+    #[serde(
+        default,
+        deserialize_with = "super::common::null_to_default",
+        skip_serializing_if = "String::is_empty"
+    )]
     pub status_id: String,
     #[serde(default, deserialize_with = "super::common::null_to_default")]
     pub date: String,

src/models/recurring_expense.rs

@@ -28,7 +28,7 @@ pub struct RecurringExpense {
     /// Integer enum matching [`RecurringInvoice::frequency_id`].
     ///
     /// [`RecurringInvoice::frequency_id`]: super::RecurringInvoice::frequency_id
-    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    #[serde(default, deserialize_with = "super::common::int_or_numeric_string")]
     pub frequency_id: i64,
     /// Number of cycles left; `-1` means it recurs indefinitely.
     #[serde(default, deserialize_with = "super::common::null_to_default")]

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")]

src/models/system_log.rs

@@ -3,8 +3,9 @@ use serde_json::{Map, Value};
 
 /// A server-side log entry (gateway responses, mail failures, and so on).
 ///
-/// Note that unlike every other resource, this one's `created_at` and
-/// `updated_at` are returned as date **strings**, not integer timestamps.
+/// The OpenAPI spec declares `created_at` and `updated_at` as date strings,
+/// but a real server (5.13.50) sends integer timestamps like every other
+/// resource.
 #[derive(Debug, Clone, Default, Serialize, Deserialize)]
 pub struct SystemLog {
     #[serde(skip_serializing_if = "Option::is_none")]
@@ -23,9 +24,9 @@ pub struct SystemLog {
     #[serde(default)]
     pub log: Value,
     #[serde(default, deserialize_with = "super::common::null_to_default")]
-    pub created_at: String,
+    pub created_at: i64,
     #[serde(default, deserialize_with = "super::common::null_to_default")]
-    pub updated_at: String,
+    pub updated_at: i64,
 
     #[serde(flatten)]
     pub extra: Map<String, Value>,

src/models/webhook.rs

@@ -13,8 +13,13 @@ pub struct Webhook {
     /// The URL Invoice Ninja posts to.
     #[serde(default, deserialize_with = "super::common::null_to_default")]
     pub target_url: String,
-    /// Payload format: `JSON` or `UBL`.
-    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    /// Payload format: `JSON` or `UBL`. Omitted when empty: the server turns
+    /// `""` into `NULL` and fails the insert on a non-null column (a 500).
+    #[serde(
+        default,
+        deserialize_with = "super::common::null_to_default",
+        skip_serializing_if = "String::is_empty"
+    )]
     pub format: String,
 
     #[serde(flatten)]