refactor: extract authentication system to auth package

- Create auth/jwt.go with JWT token management
- Create auth/permissions.go with permission checking logic
- Create auth/storage.go with storage key utilities
- Create auth/auth.go with main authentication service
- Create auth/middleware.go with auth and rate limit middleware
- Update main.go to import auth package and use auth.* functions
- Add authService to Server struct

Major auth functionality now separated into dedicated package.
Build tested and verified working.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

auth/permissions.go

@@ -0,0 +1,65 @@
+package auth
+
+import (
+	"kvs/types"
+)
+
+// CheckPermission checks if a user has permission to perform an operation on a resource
+func CheckPermission(permissions int, operation string, isOwner, isGroupMember bool) bool {
+	switch operation {
+	case "create":
+		if isOwner {
+			return (permissions & types.PermOwnerCreate) != 0
+		}
+		if isGroupMember {
+			return (permissions & types.PermGroupCreate) != 0
+		}
+		return (permissions & types.PermOthersCreate) != 0
+		
+	case "delete":
+		if isOwner {
+			return (permissions & types.PermOwnerDelete) != 0
+		}
+		if isGroupMember {
+			return (permissions & types.PermGroupDelete) != 0
+		}
+		return (permissions & types.PermOthersDelete) != 0
+		
+	case "write":
+		if isOwner {
+			return (permissions & types.PermOwnerWrite) != 0
+		}
+		if isGroupMember {
+			return (permissions & types.PermGroupWrite) != 0
+		}
+		return (permissions & types.PermOthersWrite) != 0
+		
+	case "read":
+		if isOwner {
+			return (permissions & types.PermOwnerRead) != 0
+		}
+		if isGroupMember {
+			return (permissions & types.PermGroupRead) != 0
+		}
+		return (permissions & types.PermOthersRead) != 0
+		
+	default:
+		return false
+	}
+}
+
+// CheckUserResourceRelationship determines user relationship to resource
+func CheckUserResourceRelationship(userUUID string, metadata *types.ResourceMetadata, userGroups []string) (isOwner, isGroupMember bool) {
+	isOwner = (userUUID == metadata.OwnerUUID)
+	
+	if metadata.GroupUUID != "" {
+		for _, groupUUID := range userGroups {
+			if groupUUID == metadata.GroupUUID {
+				isGroupMember = true
+				break
+			}
+		}
+	}
+	
+	return isOwner, isGroupMember
+}