fix(db): pin the tenant scope to one connection (coreci-chat#14)

packages/db/src/db-client.ts

@@ -16,4 +16,14 @@ export interface DbClient {
   query<T = Record<string, unknown>>(text: string, params?: unknown[]): Promise<QueryResult<T>>;
   /** Run multi-statement SQL (DDL, migrations). No params. Use for migrations only. */
   exec(text: string): Promise<void>;
+  /**
+   * Run `fn` inside one transaction on ONE connection.
+   *
+   * A pooled client can hand out a different connection per statement, so a
+   * transaction-local setting (the tenant scope `SET LOCAL app.tenant_id`)
+   * only holds if every statement of the transaction shares a connection.
+   * Optional: a client that cannot pin a connection omits it, and withTenant
+   * keeps its own BEGIN/COMMIT path as the fallback.
+   */
+  transaction?<T>(fn: (client: DbClient) => Promise<T>): Promise<T>;
 }