Stabilize issue-project e2e test, increase timeout factor (#37297)

1. stabilize flaky e2e test from
https://github.com/go-gitea/gitea/commit/2f5b5a9e9c32e6042f1f06f1b112a795267d6955
2. increase ci timeout factor to 4 as 3 was not enough
3. add a `e2e` category to files-changed so e2e-test-only changes
trigger ci

---
This PR was written with the help of Claude Opus 4.7

---------

Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>

.github/workflows/pull-e2e-tests.yml

@@ -14,7 +14,7 @@ jobs:
       contents: read
 
   test-e2e:
-    if: needs.files-changed.outputs.backend == 'true' || needs.files-changed.outputs.frontend == 'true'
+    if: needs.files-changed.outputs.backend == 'true' || needs.files-changed.outputs.frontend == 'true' || needs.files-changed.outputs.e2e == 'true'
     needs: files-changed
     runs-on: ubuntu-latest
     permissions: