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>

packages/db/src/schema/users.ts

@@ -30,6 +30,7 @@ export const users = pgTable("users", {
   stripeCustomerId: text("stripe_customer_id").unique(),
   unitPref: unitPrefEnum("unit_pref").notNull().default("metric"),
   locale: text("locale").notNull().default("en"),
+  twoFactorEnabled: boolean("two_factor_enabled").notNull().default(false),
   createdAt: timestamp("created_at").notNull().defaultNow(),
   updatedAt: timestamp("updated_at").notNull().defaultNow(),
 });
@@ -71,6 +72,16 @@ export const verifications = pgTable("verifications", {
   updatedAt: timestamp("updated_at").notNull().defaultNow(),
 });
 
+// Better Auth's twoFactor plugin table — secret/backupCodes are encrypted by
+// the plugin before storage, not plaintext.
+export const twoFactors = pgTable("two_factors", {
+  id: text("id").primaryKey(),
+  userId: text("user_id").notNull().references(() => users.id, { onDelete: "cascade" }),
+  secret: text("secret").notNull(),
+  backupCodes: text("backup_codes").notNull(),
+  verified: boolean("verified").notNull().default(true),
+});
+
 export const userFollows = pgTable("user_follows", {
   followerId: text("follower_id").notNull().references(() => users.id, { onDelete: "cascade" }),
   followingId: text("following_id").notNull().references(() => users.id, { onDelete: "cascade" }),