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/handler.go

@@ -11,6 +11,7 @@ import (
 
 	"github.com/gin-gonic/gin"
 	"github.com/router-for-me/CLIProxyAPI/v6/internal/config"
+	"github.com/router-for-me/CLIProxyAPI/v6/internal/usage"
 	coreauth "github.com/router-for-me/CLIProxyAPI/v6/sdk/cliproxy/auth"
 	"golang.org/x/crypto/bcrypt"
 )
@@ -29,11 +30,18 @@ type Handler struct {
 	attemptsMu     sync.Mutex
 	failedAttempts map[string]*attemptInfo // keyed by client IP
 	authManager    *coreauth.Manager
+	usageStats     *usage.RequestStatistics
 }
 
 // NewHandler creates a new management handler instance.
 func NewHandler(cfg *config.Config, configFilePath string, manager *coreauth.Manager) *Handler {
-	return &Handler{cfg: cfg, configFilePath: configFilePath, failedAttempts: make(map[string]*attemptInfo), authManager: manager}
+	return &Handler{
+		cfg:            cfg,
+		configFilePath: configFilePath,
+		failedAttempts: make(map[string]*attemptInfo),
+		authManager:    manager,
+		usageStats:     usage.GetRequestStatistics(),
+	}
 }
 
 // SetConfig updates the in-memory config reference when the server hot-reloads.
@@ -42,6 +50,9 @@ func (h *Handler) SetConfig(cfg *config.Config) { h.cfg = cfg }
 // SetAuthManager updates the auth manager reference used by management endpoints.
 func (h *Handler) SetAuthManager(manager *coreauth.Manager) { h.authManager = manager }
 
+// SetUsageStatistics allows replacing the usage statistics reference.
+func (h *Handler) SetUsageStatistics(stats *usage.RequestStatistics) { h.usageStats = stats }
+
 // Middleware enforces access control for management endpoints.
 // All requests (local and remote) require a valid management key.
 // Additionally, remote access requires allow-remote-management=true.

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})
+}