refactor: extract shared login credential verification into auth/login-core

The username normalization, dummy-hash constant, password check and
email-verification gate were duplicated between precheckLogin and the
NextAuth credentials authorize handler. Move them into a single
login-core module so both paths share one source of truth and stay
consistent.

src/actions/auth-precheck.ts

@@ -1,11 +1,14 @@
 "use server";
 
-import { env } from "@/env";
-import { checkLogin } from "@/lib/auth/password";
-import { queryPreparedOne } from "@/lib/db";
+import {
+	getLoginUser,
+	isEmailUnverified,
+	normalizeLoginInput,
+	runDummyHashCheck,
+	verifyLoginPassword,
+} from "@/lib/auth/login-core";
 import { clientIp, rateLimit } from "@/lib/rate-limit";
 import { captchaConfig, verifyCaptcha } from "@/lib/services/captcha";
-import { siteSettings } from "@/lib/services/site-settings";
 
 export type PrecheckResult =
 	| "ok"
@@ -24,10 +27,7 @@ export async function precheckLogin(
 	password: string,
 	captchaToken?: string | null,
 ): Promise<PrecheckResult> {
-	const u = String(username ?? "")
-		.normalize("NFC")
-		.trim();
-	const p = String(password ?? "").normalize("NFC");
+	const { username: u, password: p } = normalizeLoginInput(username, password);
 	if (!u || !p) return "invalid";
 
 	const ip = await clientIp();
@@ -38,49 +38,17 @@ export async function precheckLogin(
 		if (!(await verifyCaptcha(captchaToken ?? null, ip))) return "captcha";
 	}
 
-	let user: {
-		password: string;
-		twoFactorConfirmedAt: Date | null;
-		mail: string | null;
-		mailVerified: string;
-	} | null;
-	try {
-		user = await queryPreparedOne<{
-			password: string;
-			twoFactorConfirmedAt: Date | null;
-			mail: string | null;
-			mailVerified: string;
-		}>(
-			`SELECT password, two_factor_confirmed_at AS twoFactorConfirmedAt,
-			        mail, mail_verified AS mailVerified
-			 FROM users WHERE username = ? LIMIT 1`,
-			[u],
-		);
-	} catch {
-		return "invalid";
-	}
+	const user = await getLoginUser(u);
 	if (!user) {
 		// Prevent timing-based enumeration: always run a dummy hash check.
-		await checkLogin(
-			p,
-			"$2y$12$abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZabcd",
-			{
-				convertPasswords: false,
-			},
-		);
+		await runDummyHashCheck(p);
 		return "invalid";
 	}
 
-	const res = await checkLogin(p, user.password, {
-		convertPasswords: env.CONVERT_PASSWORDS,
-	});
+	const res = await verifyLoginPassword(user, p);
 	if (!res.valid) return "invalid";
 
-	if (
-		(await siteSettings.getBool("require_email_verification", false)) &&
-		user.mail &&
-		user.mailVerified !== "1"
-	) {
+	if (await isEmailUnverified(user)) {
 		return "unverified";
 	}