feat(notify): convert email lists to NotifyEmailEntry struct with toggle support

- Change balance_notify_extra_emails and account_quota_notify_emails
  from []string to []NotifyEmailEntry{email, disabled, verified}
- Add per-email enable/disable toggle for both user and admin notifications
- Add PUT /user/notify-email/toggle API endpoint
- Fix critical bug: API key auth cache snapshot missing balance notify
  fields (Email, Username, BalanceNotifyEnabled, etc.), causing
  notifications to never fire on cached request paths
- Bump cache snapshot version 3→4 to invalidate stale entries
- Add SQL migration 104 to convert old format data
- Backward compatible: parseNotifyEmails auto-detects old/new format
- User balance notify: max 3 emails (primary + 2 extra)
- Admin quota notify: unlimited emails, each with toggle

backend/migrations/104_migrate_notify_emails_to_struct.sql

@@ -0,0 +1,35 @@
+-- Migrate notification email lists from old []string format to new []NotifyEmailEntry format
+-- Old: ["a@x.com", "b@x.com"]
+-- New: [{"email":"a@x.com","disabled":false,"verified":true}, ...]
+-- Existing emails are marked as verified=false (unverified), disabled=false (enabled)
+
+-- 1. User balance notification emails
+UPDATE users
+SET balance_notify_extra_emails = (
+  SELECT COALESCE(
+    jsonb_agg(jsonb_build_object('email', elem::text, 'disabled', false, 'verified', false)),
+    '[]'::jsonb
+  )::text
+  FROM jsonb_array_elements_text(balance_notify_extra_emails::jsonb) AS elem
+)
+WHERE balance_notify_extra_emails IS NOT NULL
+  AND balance_notify_extra_emails <> '[]'
+  AND balance_notify_extra_emails <> ''
+  AND (balance_notify_extra_emails::jsonb -> 0) IS NOT NULL
+  AND jsonb_typeof(balance_notify_extra_emails::jsonb -> 0) = 'string';
+
+-- 2. Admin account quota notification emails
+UPDATE settings
+SET value = (
+  SELECT COALESCE(
+    jsonb_agg(jsonb_build_object('email', elem::text, 'disabled', false, 'verified', false)),
+    '[]'::jsonb
+  )::text
+  FROM jsonb_array_elements_text(value::jsonb) AS elem
+)
+WHERE key = 'account_quota_notify_emails'
+  AND value IS NOT NULL
+  AND value <> '[]'
+  AND value <> ''
+  AND (value::jsonb -> 0) IS NOT NULL
+  AND jsonb_typeof(value::jsonb -> 0) = 'string';