feat: websearch quota enhancements and balance notify hint

- QuotaLimit changed to *int64 (null=unlimited, >0=limited)
- Add reset-usage endpoint (POST /admin/settings/web-search-emulation/reset-usage)
- Show quota usage in header always (collapsed and expanded)
- Add reset quota button in expanded provider view
- Quota input: empty=unlimited with ∞ placeholder, must be >0 if set
- Add email verification hint on balance notify card

backend/internal/service/websearch_config.go

@@ -24,7 +24,7 @@ type WebSearchProviderConfig struct {
 	Type             string `json:"type"`                    // websearch.ProviderTypeBrave | Tavily
 	APIKey           string `json:"api_key,omitempty"`       // secret — omitted in API responses
 	APIKeyConfigured bool   `json:"api_key_configured"`      // read-only mask
-	QuotaLimit       int64  `json:"quota_limit"`             // 0 = unlimited
+	QuotaLimit       *int64 `json:"quota_limit"`             // nil = unlimited, >0 = limited
 	SubscribedAt     *int64 `json:"subscribed_at,omitempty"` // subscription start (unix seconds); quota resets monthly
 	QuotaUsed        int64  `json:"quota_used,omitempty"`    // read-only: current usage from Redis
 	ProxyID          *int64 `json:"proxy_id"`                // optional proxy association
@@ -52,8 +52,8 @@ func validateWebSearchConfig(cfg *WebSearchEmulationConfig) error {
 		if !validProviderTypes[p.Type] {
 			return fmt.Errorf("provider[%d]: invalid type %q", i, p.Type)
 		}
-		if p.QuotaLimit < 0 {
-			return fmt.Errorf("provider[%d]: quota_limit must be >= 0", i)
+		if p.QuotaLimit != nil && *p.QuotaLimit < 0 {
+			return fmt.Errorf("provider[%d]: quota_limit must be > 0 or null", i)
 		}
 		if seen[p.Type] {
 			return fmt.Errorf("provider[%d]: duplicate type %q", i, p.Type)
@@ -299,6 +299,15 @@ func PopulateWebSearchUsage(ctx context.Context, cfg *WebSearchEmulationConfig)
 	return &out
 }
 
+// ResetWebSearchUsage deletes the Redis quota key for the given provider type.
+func ResetWebSearchUsage(ctx context.Context, providerType string) error {
+	mgr := getWebSearchManager()
+	if mgr == nil {
+		return fmt.Errorf("web search manager not initialized")
+	}
+	return mgr.ResetUsage(ctx, providerType)
+}
+
 // SanitizeWebSearchConfig returns a copy with api_key fields masked and quota usage populated.
 func SanitizeWebSearchConfig(ctx context.Context, cfg *WebSearchEmulationConfig) *WebSearchEmulationConfig {
 	if cfg == nil {