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)

.gitignore

@@ -0,0 +1,11 @@
+# Never commit secrets or runtime state
+*.token
+*.pem
+*.key
+.aws/
+.config/rclone/
+logs/
+db/
+repos/
+*.bundle
+*.db.gz