fix: make the Hub backup scripts runnable on the Linux host

Found by running deploy/hub/backup.sh on the production VM:

- With core.autocrlf=true, `git archive` on the Windows release machine
  shipped the .sh files with CRLF and `sh` aborted on line 4
  ("set: Illegal option -"): backup, restore and seed never ran.
  .gitattributes now forces *.sh to LF.
- Docker creates the bind-mounted backups/ directory as root, so the
  host-side retention `rm` would fail once the 15th dump existed.
  Retention now runs inside the postgres container.

tests/test_deploy_scripts.py covers both.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitattributes

@@ -0,0 +1,4 @@
+# Shell scripts run on the Linux Hub host. With core.autocrlf=true on the
+# Windows release machine, `git archive`/checkout would otherwise ship them
+# with CRLF and `sh` rejects them ("set: Illegal option -").
+*.sh text eol=lf