Website feedback: fixes from review

- Rate limiting fails open on KV errors instead of dropping feedback
- Break owner/repo#1, GH-1 and github.com references in user text
- Time the form with the browser's monotonic clock, not wall-clock
- Serialize lgtm approvals and rebase before pushing

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.github/workflows/approve-contributor.yml

@@ -7,6 +7,11 @@ on:
   issue_comment:
     types: [created]
 
+# One approval at a time, so two lgtm replies can't race on the same commit.
+concurrency:
+  group: approve-contributor
+  cancel-in-progress: false
+
 jobs:
   approve:
     runs-on: ubuntu-latest
@@ -183,6 +188,8 @@ jobs:
           git config user.email "github-actions[bot]@users.noreply.github.com"
           git add .github/APPROVED_CONTRIBUTORS
           git diff --staged --quiet || git commit -m "chore: approve contributors from issue #${{ github.event.issue.number }}"
+          # main may have moved since checkout; replay the approval on top of it.
+          git pull --rebase origin "${{ github.event.repository.default_branch }}"
           git push
 
       - name: Comment on issue