fix: address audit findings for websearch, email verification, and pricing

- Fix websearch provider failover: proxy error from provider-specific proxy
  now continues to next provider instead of aborting the entire loop
- Fix SMTP failure locking users out: send email first, then write cache
  and increment rate counter
- Fix notify email cache key case sensitivity: normalize to lowercase
- Add OriginalPrice validation to validatePlanPatch and validatePlanRequired
- Add empty scope validation for channel pricing rules (group_ids/account_ids)
- Add platform color to account search dropdown in channel pricing rules

backend/internal/repository/email_cache.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"strings"
 	"time"
 
 	"github.com/Wei-Shaw/sub2api/internal/service"
@@ -24,8 +25,10 @@ func verifyCodeKey(email string) string {
 }
 
 // notifyVerifyKey generates the Redis key for notify email verification code.
+// Email is lowercased to prevent case-sensitive key mismatch (the business layer
+// uses strings.EqualFold for comparison).
 func notifyVerifyKey(email string) string {
-	return notifyVerifyKeyPrefix + email
+	return notifyVerifyKeyPrefix + strings.ToLower(email)
 }
 
 // passwordResetKey generates the Redis key for password reset token.