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/offer-commands.test.ts

@@ -16,6 +16,7 @@ const state = vi.hoisted(() => ({
 	rollbacks: 0,
 }));
 vi.mock("@/lib/db", async () => ({
+	rowsFrom: (await import("@/test/db-helpers")).rowsFrom,
 	...(await import("@/db/schema")),
 	db: {
 		transaction: async (fn: (tx: unknown) => Promise<unknown>) => {