Initial commit: backup + mirror automation for self-hosted Gitea

Includes:
- gitea-backups/bin/backup.sh (per-push bundle + DB snapshot to local + S3)
- gitea-backups/bin/install-hooks.sh (idempotent post-receive shim installer)
- gitea-backups/bin/retention.sh (count-based retention: keep newest 7 dates)
- gitea-mirror/bin/auto-mirror.sh (Gitea -> GitHub push mirror automation,
  hardened against Gitea outages)
- crontab.txt (reference for the 3 cron entries)
- README.md (architecture, layout, bootstrap)

crontab.txt

@@ -0,0 +1,12 @@
+# Crontab reference for the Gitea automation.
+# Install with:  crontab crontab.txt
+# Verify with:   crontab -l
+
+# Every minute: ensure backup post-receive hook is installed in every Gitea repo
+* * * * * /home/ubuntu/gitea-backups/bin/install-hooks.sh --quiet >> /home/ubuntu/gitea-backups/logs/install-hooks.log 2>&1
+
+# Every minute: ensure every Gitea repo has a GitHub push-mirror configured
+* * * * * /home/ubuntu/gitea-mirror/bin/auto-mirror.sh --quiet >> /home/ubuntu/gitea-mirror/logs/cron.log 2>&1
+
+# Daily at 03:00 UTC: keep newest 7 calendar dates of backups (S3 + local)
+0 3 * * * /home/ubuntu/gitea-backups/bin/retention.sh --quiet >> /home/ubuntu/gitea-backups/logs/retention.log 2>&1