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/pkg/websearch/manager.go

@@ -111,9 +111,18 @@ func (m *Manager) SearchWithBestProvider(ctx context.Context, req SearchRequest)
 			}
 			if isProxyError(err) {
 				m.markProxyUnavailable(ctx, cfg, req.ProxyURL)
-				slog.Warn("websearch: proxy error, marking unavailable",
+				if req.ProxyURL != "" {
+					// Account-level proxy is shared by all providers — no point
+					// trying others with the same broken proxy; signal account switch.
+					slog.Warn("websearch: account proxy error, aborting failover",
+						"provider", cfg.Type, "error", err)
+					return nil, "", fmt.Errorf("%w: %s", ErrProxyUnavailable, err.Error())
+				}
+				// Provider-specific proxy failed — try the next provider which
+				// may use a different (or no) proxy.
+				slog.Warn("websearch: provider proxy error, trying next provider",
 					"provider", cfg.Type, "error", err)
-				return nil, "", fmt.Errorf("%w: %s", ErrProxyUnavailable, err.Error())
+				continue
 			}
 			slog.Warn("websearch: provider search failed",
 				"provider", cfg.Type, "error", err)