Add category.order to reorder siblings under a parent.

Sets each direct child's sort to its index in the given categoryIds,
scoped by parentId. Validates the set matches the current children.

src/store/category/order.js

@@ -0,0 +1,62 @@
+import {
+  LIST_CHILD_IDS,
+  UPDATE_SORT,
+} from '../../sql/category.js';
+import request from '../request.js';
+import resolveCategoryAccess from './resolveCategoryAccess.js';
+
+export default function order(pool) {
+  const access = resolveCategoryAccess(pool);
+
+  return async ({ googleAuthId, parentId, categoryIds }) => {
+    if (!Array.isArray(categoryIds)) {
+      throw new Error('Invalid category order');
+    }
+
+    // Auth + ensures parentId is a real category of the user's company.
+    await access(googleAuthId, parentId);
+
+    const children = await pool
+      .request()
+      .input('parentId', parentId)
+      .query(LIST_CHILD_IDS);
+
+    const existing = new Set(children.recordset.map((r) => r.id));
+
+    if (categoryIds.length !== existing.size) {
+      throw new Error('Invalid category order');
+    }
+
+    const seen = new Set();
+    for (const categoryId of categoryIds) {
+      if (!Number.isFinite(categoryId) || !existing.has(categoryId) || seen.has(categoryId)) {
+        throw new Error('Invalid category order');
+      }
+      seen.add(categoryId);
+    }
+
+    const tx = pool.transaction();
+    await tx.begin();
+
+    try {
+      for (let sort = 0; sort < categoryIds.length; sort++) {
+        await request(pool, tx)
+          .input('id', categoryIds[sort])
+          .input('parentId', parentId)
+          .input('sort', sort)
+          .query(UPDATE_SORT);
+      }
+
+      await tx.commit();
+
+      return { categoryIds };
+    } catch (err) {
+      try {
+        await tx.rollback();
+      } catch {
+        // ignore
+      }
+      throw err;
+    }
+  };
+}