Fix workflow race condition with concurrency control and pull-rebase

- Add concurrency group to prevent parallel workflow runs
- Pull and rebase before pushing to handle remote changes
- Auto-regenerate files if rebase fails to resolve conflicts

.gitea/workflows/update-calendar.yaml

@@ -13,6 +13,10 @@ on:
     - cron: '0 0 * * *'
   workflow_dispatch:
 
+concurrency:
+  group: update-calendar
+  cancel-in-progress: false
+
 jobs:
   update-calendar:
     runs-on: ubuntu-latest
@@ -44,6 +48,16 @@ jobs:
             git commit -m "Auto-update calendar files
 
           🤖 Generated with automation"
-            git pull --rebase origin main
+            # Pull latest changes and rebase our commit on top
+            git pull --rebase origin main || {
+              # If rebase fails, regenerate files to resolve conflicts
+              git rebase --abort
+              git pull origin main
+              python generate_calendar.py
+              git add calendar.ics README.md
+              git commit -m "Auto-update calendar files
+
+          🤖 Generated with automation"
+            }
             git push
           fi