feat: two-factor authentication (TOTP + backup codes)

Uses better-auth's built-in twoFactor plugin rather than hand-rolling
TOTP — adds the two_factors table and users.twoFactorEnabled, wires
the server/client plugins (allowPasswordless: true so OAuth-only
accounts aren't locked out of managing 2FA), and adds a custom rate
limit for the verify endpoints (5/min — far stricter than the default
100/10s, since a 6-digit code has a much smaller keyspace than a
password).

New Settings → Security section walks through enable (password
confirm -> QR + backup codes -> confirm a live code before it's
actually turned on, per the plugin's default flow) and disable. New
/verify-2fa page handles the post-password mid-login step, with a
backup-code fallback. Verified live end-to-end: enable, confirm,
forced re-auth on next sign-in, TOTP accepted, backup code accepted
(single-use), disable.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/lib/changelog.ts

@@ -1,5 +1,5 @@
 // Mirrors CHANGELOG.md at the repo root — update both together.
-export const APP_VERSION = "0.22.0";
+export const APP_VERSION = "0.23.0";
 
 export type ChangelogEntry = {
   version: string;
@@ -11,6 +11,13 @@ export type ChangelogEntry = {
 };
 
 export const CHANGELOG: ChangelogEntry[] = [
+  {
+    version: "0.23.0",
+    date: "2026-07-13 23:06",
+    added: [
+      "**Two-factor authentication**: turn it on in Settings → Security to require a code from an authenticator app when signing in, with backup codes in case you lose access to it.",
+    ],
+  },
   {
     version: "0.22.0",
     date: "2026-07-13 22:49",