feat: Implement two-factor authentication (2FA) with dedicated endpoints for setup, enabling, disabling, verification, and backup code management.

prisma/schema.prisma

@@ -83,6 +83,12 @@ model User {
   facebookId        String?      @unique
   twitterId         String?      @unique
 
+  // Two-Factor Authentication
+  twoFactorEnabled    Boolean   @default(false)
+  twoFactorSecret     String?   // Encrypted TOTP secret
+  twoFactorBackupCodes String?  // JSON array of hashed backup codes
+  twoFactorVerifiedAt DateTime? // When 2FA was enabled
+
   // Timestamps
   createdAt         DateTime     @default(now())
   updatedAt         DateTime     @updatedAt