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/internal/service/notify_email_entry.go

@@ -0,0 +1,107 @@
+package service
+
+import (
+	"encoding/json"
+	"strings"
+)
+
+// NotifyEmailEntry represents a notification email with enable/disable and verification state.
+// Email="" is a placeholder for the "primary email" (user's registration email or first admin email).
+type NotifyEmailEntry struct {
+	Email    string `json:"email"`
+	Disabled bool   `json:"disabled"`
+	Verified bool   `json:"verified"`
+}
+
+// parseNotifyEmails parses a JSON string into []NotifyEmailEntry.
+// It auto-detects the format:
+//   - Old format ["email1","email2"] → converted to [{email, disabled:false, verified:true}, ...]
+//   - New format [{email,disabled,verified}, ...] → parsed directly
+//
+// Returns nil on empty/invalid input.
+func ParseNotifyEmails(raw string) []NotifyEmailEntry {
+	raw = strings.TrimSpace(raw)
+	if raw == "" || raw == "[]" {
+		return nil
+	}
+
+	// Try parsing as new format first (array of objects)
+	var entries []NotifyEmailEntry
+	if err := json.Unmarshal([]byte(raw), &entries); err == nil && len(entries) > 0 {
+		// Verify it's actually the new format by checking the first element
+		// json.Unmarshal into []NotifyEmailEntry succeeds even for ["string"]
+		// because it tries to fit "string" into NotifyEmailEntry and gets zero values.
+		// We need to detect old format explicitly.
+		if !isOldStringArrayFormat(raw) {
+			return entries
+		}
+	}
+
+	// Try parsing as old format (array of strings)
+	var emails []string
+	if err := json.Unmarshal([]byte(raw), &emails); err == nil {
+		result := make([]NotifyEmailEntry, 0, len(emails))
+		for _, e := range emails {
+			e = strings.TrimSpace(e)
+			if e != "" {
+				result = append(result, NotifyEmailEntry{
+					Email:    e,
+					Disabled: false,
+					Verified: false, // Old format emails default to unverified
+				})
+			}
+		}
+		return result
+	}
+
+	return nil
+}
+
+// isOldStringArrayFormat checks if the JSON is a string array like ["email1","email2"].
+func isOldStringArrayFormat(raw string) bool {
+	var arr []json.RawMessage
+	if err := json.Unmarshal([]byte(raw), &arr); err != nil || len(arr) == 0 {
+		return false
+	}
+	// Check if first element starts with a quote (string) vs { (object)
+	first := strings.TrimSpace(string(arr[0]))
+	return len(first) > 0 && first[0] == '"'
+}
+
+// marshalNotifyEmails serializes []NotifyEmailEntry to JSON string.
+func MarshalNotifyEmails(entries []NotifyEmailEntry) string {
+	if len(entries) == 0 {
+		return "[]"
+	}
+	data, err := json.Marshal(entries)
+	if err != nil {
+		return "[]"
+	}
+	return string(data)
+}
+
+// filterEnabledEmails returns only non-disabled email addresses from entries.
+// Empty email placeholders are skipped (caller should resolve them separately).
+func FilterEnabledEmails(entries []NotifyEmailEntry) []string {
+	var result []string
+	for _, e := range entries {
+		if e.Disabled {
+			continue
+		}
+		email := strings.TrimSpace(e.Email)
+		if email != "" {
+			result = append(result, email)
+		}
+	}
+	return result
+}
+
+// isPrimaryDisabled checks if the primary email placeholder (email="") exists and is disabled.
+func IsPrimaryDisabled(entries []NotifyEmailEntry) bool {
+	for _, e := range entries {
+		if e.Email == "" {
+			return e.Disabled
+		}
+	}
+	return false // No primary placeholder = not disabled
+}