feat(usage): add in-memory usage statistics tracking and API endpoint

- Introduced in-memory request statistics aggregation in `LoggerPlugin`.
- Added new structures for detailed metrics collection (e.g., token breakdown, request success/failure).
- Implemented `/usage` management API endpoint for retrieving aggregated statistics.
- Updated management handlers to support the new usage statistics functionality.
- Enhanced documentation to describe the usage metrics API.

internal/api/handlers/management/usage.go

@@ -0,0 +1,17 @@
+package management
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	"github.com/router-for-me/CLIProxyAPI/v6/internal/usage"
+)
+
+// GetUsageStatistics returns the in-memory request statistics snapshot.
+func (h *Handler) GetUsageStatistics(c *gin.Context) {
+	var snapshot usage.StatisticsSnapshot
+	if h != nil && h.usageStats != nil {
+		snapshot = h.usageStats.Snapshot()
+	}
+	c.JSON(http.StatusOK, gin.H{"usage": snapshot})
+}