fix: validate and clamp container log tail parameter (closes #24)

- Add sanitizeTail() helper that validates tail is numeric and positive
- Clamp values to max 500
- Default to 500 when empty, non-numeric, zero, or negative
- Add comprehensive test cases

internal/handlers/app.go

@@ -373,6 +373,28 @@ func (h *Handlers) HandleAppDeployments() http.HandlerFunc {
 // defaultLogTail is the default number of log lines to fetch.
 const defaultLogTail = "500"
 
+// maxLogTail is the maximum allowed value for the tail parameter.
+const maxLogTail = 500
+
+// sanitizeTail validates and clamps the tail query parameter.
+// It returns a numeric string clamped to maxLogTail, or the default if invalid.
+func sanitizeTail(raw string) string {
+	if raw == "" {
+		return defaultLogTail
+	}
+
+	n, err := strconv.Atoi(raw)
+	if err != nil || n < 1 {
+		return defaultLogTail
+	}
+
+	if n > maxLogTail {
+		n = maxLogTail
+	}
+
+	return strconv.Itoa(n)
+}
+
 // HandleAppLogs returns the container logs handler.
 func (h *Handlers) HandleAppLogs() http.HandlerFunc {
 	return func(writer http.ResponseWriter, request *http.Request) {
@@ -394,10 +416,7 @@ func (h *Handlers) HandleAppLogs() http.HandlerFunc {
 			return
 		}
 
-		tail := request.URL.Query().Get("tail")
-		if tail == "" {
-			tail = defaultLogTail
-		}
+		tail := sanitizeTail(request.URL.Query().Get("tail"))
 
 		logs, logsErr := h.docker.ContainerLogs(
 			request.Context(),