Reject path traversal in deploy log download handler (closes #177)

gosec flagged G703 (path traversal via taint analysis) on the log
download handler because the served path derives from a URL parameter.
Open the log file through an os.Root confined to the deploy log
directory instead of passing the path to http.ServeFile; Root.Open
rejects any path that escapes the root, so traversal attempts return
404. Serve the opened file with http.ServeContent. Adds GetLogDir on
the deploy service.

The traversal regression test plants a sentinel file at the location a
traversal-shaped app name resolves to (outside the log directory) and
asserts the handler returns 404 without leaking the sentinel, so it
fails if the os.Root guard is removed. Keeps the legitimate-download
test.

Model: opus-4-8

internal/service/deploy/deploy.go

@@ -294,6 +294,12 @@ func (svc *Service) GetLogFilePath(
 	return filepath.Join(svc.config.DataDir, "logs", hostname, app.Name, filename)
 }
 
+// GetLogDir returns the root directory under which all deployment log
+// files live. Paths returned by GetLogFilePath are always inside it.
+func (svc *Service) GetLogDir() string {
+	return filepath.Join(svc.config.DataDir, "logs")
+}
+
 // HasActiveDeploy returns true if there is an active deployment for the given app.
 func (svc *Service) HasActiveDeploy(appID string) bool {
 	_, ok := svc.activeDeploys.Load(appID)