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/repository/email_cache.go

@@ -3,6 +3,7 @@ package repository
 import (
 	"context"
 	"encoding/json"
+	"fmt"
 	"time"
 
 	"github.com/Wei-Shaw/sub2api/internal/service"
@@ -10,10 +11,11 @@ import (
 )
 
 const (
-	verifyCodeKeyPrefix          = "verify_code:"
-	notifyVerifyKeyPrefix        = "notify_verify:"
-	passwordResetKeyPrefix       = "password_reset:"
-	passwordResetSentAtKeyPrefix = "password_reset_sent:"
+	verifyCodeKeyPrefix            = "verify_code:"
+	notifyVerifyKeyPrefix          = "notify_verify:"
+	passwordResetKeyPrefix         = "password_reset:"
+	passwordResetSentAtKeyPrefix   = "password_reset_sent:"
+	notifyCodeUserRateKeyPrefix    = "notify_code_user_rate:"
 )
 
 // verifyCodeKey generates the Redis key for email verification code.
@@ -141,3 +143,31 @@ func (c *emailCache) DeleteNotifyVerifyCode(ctx context.Context, email string) e
 	key := notifyVerifyKey(email)
 	return c.rdb.Del(ctx, key).Err()
 }
+
+// User-level rate limiting for notify email verification codes
+
+func notifyCodeUserRateKey(userID int64) string {
+	return notifyCodeUserRateKeyPrefix + fmt.Sprintf("%d", userID)
+}
+
+func (c *emailCache) IncrNotifyCodeUserRate(ctx context.Context, userID int64, window time.Duration) (int64, error) {
+	key := notifyCodeUserRateKey(userID)
+	count, err := c.rdb.Incr(ctx, key).Result()
+	if err != nil {
+		return 0, err
+	}
+	// Always set TTL (idempotent) to avoid orphan keys if process crashes between INCR and EXPIRE.
+	if err := c.rdb.Expire(ctx, key, window).Err(); err != nil {
+		return count, fmt.Errorf("expire notify code rate key: %w", err)
+	}
+	return count, nil
+}
+
+func (c *emailCache) GetNotifyCodeUserRate(ctx context.Context, userID int64) (int64, error) {
+	key := notifyCodeUserRateKey(userID)
+	count, err := c.rdb.Get(ctx, key).Int64()
+	if err != nil {
+		return 0, err
+	}
+	return count, nil
+}