fix: audit fixes for websearch, notifications, and channel pricing

P0: fix wildcard matching test assertion (config order, not longest prefix)
P0: add TotalRecharged to auth cache snapshot (v5) for percentage threshold
P1: move pricing rules into per-platform sections in ChannelsView
P1: populate account name cache when editing existing channel rules
P1: sanitize email subject headers to prevent SMTP injection
P1: make Redis INCR+EXPIRE idempotent for rate limiting
P1: deep copy FeaturesConfig in Channel.Clone()
P2: clean up stale email="" placeholder comments
P2: replace log.Printf with slog in email_service.go

backend/internal/handler/dto/notify_email_entry.go

@@ -3,7 +3,7 @@ package dto
 import "github.com/Wei-Shaw/sub2api/internal/service"
 
 // 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).
+// All emails are user-managed; maximum 3 entries per user.
 type NotifyEmailEntry struct {
 	Email    string `json:"email"`
 	Disabled bool   `json:"disabled"`

backend/internal/handler/user_handler.go

@@ -217,7 +217,7 @@ func (h *UserHandler) RemoveNotifyEmail(c *gin.Context) {
 
 // ToggleNotifyEmailRequest represents the request to toggle a notify email's disabled state
 type ToggleNotifyEmailRequest struct {
-	Email    string `json:"email"`    // empty string for primary email placeholder
+	Email    string `json:"email" binding:"required,email"`
 	Disabled bool   `json:"disabled"`
 }