feat: Implement notification preferences management for both users and agents.

prisma/schema.prisma

@@ -102,6 +102,9 @@ model User {
   twoFactorBackupCodes String?  // JSON array of hashed backup codes
   twoFactorVerifiedAt DateTime? // When 2FA was enabled
 
+  // User Preferences
+  notificationPreferences Json? // { email: {...}, push: {...} }
+
   // Timestamps
   createdAt         DateTime     @default(now())
   updatedAt         DateTime     @updatedAt