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/lib/admin-search-permissions.test.ts

@@ -18,7 +18,7 @@ vi.mock("@/lib/permissions", async () => ({
 	...(await import("@/lib/permission-slugs")),
 	canAccess: (_: unknown, slug: string) => state.allowed.has(slug),
 }));
-vi.mock("@/lib/db", () => {
+vi.mock("@/lib/db", async () => {
 	const tables = Object.fromEntries(
 		[
 			"User",