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>

tools/test-e2e.sh

@@ -95,10 +95,11 @@ GITEA_TEST_E2E_EMAIL="$GITEA_TEST_E2E_USER@$GITEA_TEST_E2E_DOMAIN"
   --must-change-password=false \
   --admin
 
-# timeout multiplier, CI runners are slower
+# timeout multiplier to make the tests pass on slow CI runners while using
+# factor 1 on a fast local machine like a MacBook Pro M1+
 if [ -z "${GITEA_TEST_E2E_TIMEOUT_FACTOR:-}" ]; then
   if [ -n "${CI:-}" ]; then
-    GITEA_TEST_E2E_TIMEOUT_FACTOR=3
+    GITEA_TEST_E2E_TIMEOUT_FACTOR=4
   else
     GITEA_TEST_E2E_TIMEOUT_FACTOR=1
   fi