feat: Expenses resource.

server/src/models/ExpenseCategory.js

@@ -0,0 +1,29 @@
+import { Model } from 'objection';
+import TenantModel from '@/models/TenantModel';
+
+export default class ExpenseCategory extends TenantModel {
+  /**
+   * Table name
+   */
+  static get tableName() {
+    return 'expense_transaction_categories';
+  }
+
+  /**
+   * Relationship mapping.
+   */
+  static get relationMappings() {
+    const Account = require('@/models/Account');
+    
+    return {
+      expenseAccount: {
+        relation: Model.BelongsToOneRelation,
+        modelClass: this.relationBindKnex(Account.default),
+        join: {
+          from: 'expense_transaction_categories.expenseAccountId',
+          to: 'accounts.id',
+        },
+      },
+    };
+  }
+}