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

The deploy-log download handler passed a request-derived path to
http.ServeFile, which gosec flags as G703 (path traversal via taint).
The handler now opens the log through an os.Root confined to the deploy
log directory, so any escaping path is rejected at runtime (404) and the
file is streamed with http.ServeContent. A regression test plants a
sentinel outside the log dir and asserts the traversal is refused and its
contents never served; removing the guard makes that test fail. No
//nolint used.

Model: opus-4-8

internal/handlers/handlers_test.go

@@ -42,6 +42,7 @@ type testContext struct {
 	database   *database.Database
 	authSvc    *auth.Service
 	appSvc     *app.Service
+	deploySvc  *deploy.Service
 	middleware *middleware.Middleware
 }
 
@@ -186,6 +187,7 @@ func setupTestHandlers(t *testing.T) *testContext {
 		database:   dbInstance,
 		authSvc:    authSvc,
 		appSvc:     appSvc,
+		deploySvc:  deploySvc,
 		middleware: mw,
 	}
 }