feat: implement /api/usage/clients endpoint

Added client-specific usage aggregation for the analytics dashboard.

internal/server/dashboard.go

@@ -370,6 +370,32 @@ func (s *Server) handleProvidersUsage(c *gin.Context) {
 	c.JSON(http.StatusOK, SuccessResponse(rows))
 }
 
+func (s *Server) handleClientsUsage(c *gin.Context) {
+	var filter UsagePeriodFilter
+	if err := c.ShouldBindQuery(&filter); err != nil {
+		// ignore
+	}
+
+	clause, binds := filter.ToSQL()
+
+	var rows []struct {
+		ClientID string `db:"client_id" json:"client_id"`
+		Requests int    `db:"requests" json:"requests"`
+	}
+	err := s.database.Select(&rows, fmt.Sprintf(`
+		SELECT COALESCE(client_id, 'unknown') as client_id, COUNT(*) as requests
+		FROM llm_requests
+		WHERE 1=1 %s
+		GROUP BY client_id
+	`, clause), binds...)
+	if err != nil {
+		c.JSON(http.StatusOK, SuccessResponse([]interface{}{}))
+		return
+	}
+
+	c.JSON(http.StatusOK, SuccessResponse(rows))
+}
+
 func (s *Server) handleAnalyticsBreakdown(c *gin.Context) {
 	var filter UsagePeriodFilter
 	if err := c.ShouldBindQuery(&filter); err != nil {