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/api/admin/prefixes/blacklist/route.ts

@@ -1,7 +1,7 @@
 import { sql } from "drizzle-orm";
 import { apiError, apiOk } from "@/lib/api";
 import { withAdmin } from "@/lib/api-handler";
-import { db } from "@/lib/db";
+import { db, queryRows } from "@/lib/db";
 import { PERMS } from "@/lib/permissions";
 
 interface BlacklistWord {
@@ -10,9 +10,9 @@ interface BlacklistWord {
 }
 
 export const GET = withAdmin({ permission: PERMS.PREFIXES_VIEW }, async () => {
-	const [words] = (await db.execute(
+	const words = await queryRows<BlacklistWord>(
 		sql`SELECT id, word FROM custom_prefix_blacklist ORDER BY word ASC`,
-	)) as unknown as [BlacklistWord[], unknown];
+	);
 
 	return apiOk({ words });
 });