refactor(db): DRY object SELECT columns, consistent date json; test date + all-none round-trip

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/db/src/catalog.rs

@@ -13,13 +13,9 @@ use crate::audit;
 /// The entity_type recorded in the audit log for catalogue objects.
 const ENTITY_TYPE: &str = "object";
 
-const SELECT_OBJECT_BY_ID: &str = "SELECT id, object_number, object_name, number_of_objects, brief_description, \
-     current_location, current_owner, recorder, recording_date, visibility, \
-     created_at, updated_at FROM object WHERE id = $1";
-
-const SELECT_OBJECTS_ORDERED: &str = "SELECT id, object_number, object_name, number_of_objects, brief_description, \
-     current_location, current_owner, recorder, recording_date, visibility, \
-     created_at, updated_at FROM object ORDER BY object_number";
+const OBJECT_COLUMNS: &str = "id, object_number, object_name, number_of_objects, \
+     brief_description, current_location, current_owner, recorder, recording_date, \
+     visibility, created_at, updated_at";
 
 /// Create an object and record a `created` audit entry, both on `conn`
 /// (pass a transaction connection `&mut *tx` so they commit atomically).
@@ -74,7 +70,9 @@ pub async fn object_by_id<'e, E>(
 where
     E: sqlx::PgExecutor<'e>,
 {
-    let row = sqlx::query(SELECT_OBJECT_BY_ID)
+    let sql = format!("SELECT {OBJECT_COLUMNS} FROM object WHERE id = $1");
+
+    let row = sqlx::query(&sql)
         .bind(id.to_uuid())
         .fetch_optional(executor)
         .await?;
@@ -88,9 +86,9 @@ where
     E: sqlx::PgExecutor<'e>,
 {
     // TODO: add LIMIT/keyset pagination before exposing this via the API.
-    let rows = sqlx::query(SELECT_OBJECTS_ORDERED)
-        .fetch_all(executor)
-        .await?;
+    let sql = format!("SELECT {OBJECT_COLUMNS} FROM object ORDER BY object_number");
+
+    let rows = sqlx::query(&sql).fetch_all(executor).await?;
 
     rows.into_iter().map(map_object).collect()
 }
@@ -137,17 +135,14 @@ fn field_values(input: &ObjectInput) -> Vec<(&'static str, Option<Value>)> {
             input.current_owner.as_ref().map(|v| json!(v)),
         ),
         ("recorder", input.recorder.as_ref().map(|v| json!(v))),
-        (
-            "recording_date",
-            input
-                .recording_date
-                .and_then(|d| serde_json::to_value(d).ok()),
-        ),
+        ("recording_date", input.recording_date.map(|d| json!(d))),
         ("visibility", Some(json!(input.visibility.as_str()))),
     ]
 }
 
 /// Audit changes for a newly created object: every set field as an `after` value.
+/// Unset (`None`) optional fields are omitted — absence is conveyed by their not
+/// appearing, consistent with `FieldChange`'s `None`-means-no-value convention.
 fn creation_changes(input: &ObjectInput) -> Vec<FieldChange> {
     field_values(input)
         .into_iter()