feat: support merchant team roles and permissions

backend/internal/database/migrations/007_merchant_roles.sql

@@ -0,0 +1,16 @@
+-- 商户自定义员工角色。role code 仅在所属商户内唯一，权限以逗号分隔保存。
+CREATE TABLE IF NOT EXISTS merchant_roles (
+    id BIGSERIAL PRIMARY KEY,
+    created_at TIMESTAMPTZ,
+    updated_at TIMESTAMPTZ,
+    merchant_id BIGINT NOT NULL,
+    code VARCHAR(64) NOT NULL,
+    name VARCHAR(64) NOT NULL,
+    permissions TEXT NOT NULL DEFAULT '',
+    status BIGINT NOT NULL DEFAULT 1
+);
+
+CREATE UNIQUE INDEX IF NOT EXISTS idx_merchant_role_code
+    ON merchant_roles (merchant_id, code);
+CREATE INDEX IF NOT EXISTS idx_merchant_roles_merchant_id
+    ON merchant_roles (merchant_id);