feat: make user limit configurable from admin user management (#18)

Store the per-instance user cap in settings (default 20) and expose
GET/PUT /admin/users/limit endpoints. The user management page now lets
admins view and update the limit without touching system settings.

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: truewhile <truewhile@users.noreply.github.com>

internal/handler/admin.go

@@ -31,7 +31,66 @@ func listUsersHandler(svc *service.Container) gin.HandlerFunc {
 		for i := range users {
 			users[i].PopulateComputedFields()
 		}
-		c.JSON(http.StatusOK, users)
+		maxUsers, err := service.LoadMaxUsers(c.Request.Context(), svc.Repo)
+		if err != nil {
+			c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
+			return
+		}
+		c.JSON(http.StatusOK, gin.H{
+			"users":        users,
+			"max_users":    maxUsers,
+			"current_users": len(users),
+		})
+	}
+}
+
+type updateUserLimitReq struct {
+	MaxUsers int `json:"max_users" binding:"required"`
+}
+
+func getUserLimitHandler(svc *service.Container) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		maxUsers, err := service.LoadMaxUsers(c.Request.Context(), svc.Repo)
+		if err != nil {
+			c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
+			return
+		}
+		currentUsers, err := svc.Repo.User.Count(c.Request.Context())
+		if err != nil {
+			c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
+			return
+		}
+		c.JSON(http.StatusOK, gin.H{
+			"max_users":     maxUsers,
+			"current_users": currentUsers,
+		})
+	}
+}
+
+func updateUserLimitHandler(svc *service.Container) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		var req updateUserLimitReq
+		if err := c.ShouldBindJSON(&req); err != nil {
+			c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+			return
+		}
+		if err := service.SaveMaxUsers(c.Request.Context(), svc.Repo, req.MaxUsers); err != nil {
+			if errors.Is(err, service.ErrInvalidMaxUsers) {
+				c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+				return
+			}
+			c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
+			return
+		}
+		currentUsers, err := svc.Repo.User.Count(c.Request.Context())
+		if err != nil {
+			c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
+			return
+		}
+		c.JSON(http.StatusOK, gin.H{
+			"max_users":     req.MaxUsers,
+			"current_users": currentUsers,
+		})
 	}
 }
 
@@ -288,7 +347,12 @@ func writeUserMutationError(c *gin.Context, svc *service.Container, err error) {
 	case errors.Is(err, service.ErrUsernameTaken):
 		c.JSON(http.StatusConflict, gin.H{"error": "username already taken"})
 	case errors.Is(err, service.ErrUserLimitReached):
-		c.JSON(http.StatusBadRequest, gin.H{"error": "user limit reached", "max_users": service.UserLimit})
+		maxUsers, loadErr := service.LoadMaxUsers(c.Request.Context(), svc.Repo)
+		if loadErr != nil {
+			c.JSON(http.StatusInternalServerError, gin.H{"error": loadErr.Error()})
+			return
+		}
+		c.JSON(http.StatusBadRequest, gin.H{"error": "user limit reached", "max_users": maxUsers})
 	default:
 		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
 	}