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/app/admin/users/_lib/load-user-overview.test.ts

@@ -7,7 +7,10 @@ const state = vi.hoisted(() => ({
 	allowed: new Set<string>(),
 	execute: vi.fn(),
 }));
-vi.mock("@/lib/db", () => ({ db: { execute: state.execute } }));
+vi.mock("@/lib/db", async () => ({
+	...(await import("@/test/db-helpers")).createDbHelpers(state.execute),
+	db: { execute: state.execute },
+}));
 vi.mock("@/lib/permissions", async () => ({
 	...(await import("@/lib/permission-slugs")),
 	canAccess: (_: unknown, permission: string) => state.allowed.has(permission),