refactor(db): typed query helpers, shared test FormData helper

Replace raw db.execute tuple casts with queryRows/rowsFrom/execResult/
affectedRows helpers from lib/db, drop redundant mysql2 casts on typed
query builders, and centralize per-test fakeForm into test/fake-form.
Update db mocks in tests so helpers resolve against mocked execute.

src/features/catalog/server/bulk-offers.test.ts

@@ -32,8 +32,8 @@ const state = vi.hoisted(() => ({
 	commits: 0,
 	rollbacks: 0,
 }));
-vi.mock("@/lib/db", () => ({
-	db: {
+vi.mock("@/lib/db", async () => {
+	const db = {
 		execute: async () => [state.pages, []],
 		transaction: async (fn: (tx: unknown) => Promise<unknown>) => {
 			const before = state.writes;
@@ -112,11 +112,16 @@ vi.mock("@/lib/db", () => ({
 				throw error;
 			}
 		},
-	},
-}));
+	};
+	return {
+		db,
+		...(await import("@/test/db-helpers")).createDbHelpers(db.execute),
+	};
+});
 
-vi.mock("@/features/operations/server", async () => {
-	const { db } = await import("@/lib/db");
+import { db } from "@/lib/db";
+
+vi.mock("@/features/operations/server", () => {
 	return {
 		runOperation: (
 			_input: unknown,