test(e2e): add dump-only-sql mixed-run + CLI contract coverage

- rename dump-only flag to --dump-only-sql across docs and tests
- update backup logic: skip files/ only for DB volumes when dumps succeed; fallback to files when dumps fail
- extend e2e helpers to support dump_only_sql
- add e2e mixed-run regression test (DB dump => no files/, non-DB => files/)
- add e2e CLI/argparse contract test (--dump-only-sql present, --dump-only rejected)
- fix e2e files test to expect file backups for non-DB volumes in dump-only-sql mode and verify restore
- update changelog + README flag table

https://chatgpt.com/share/69522d9c-ce08-800f-9070-71df3bd779ae

README.md

@@ -134,7 +134,7 @@ baudolo \
 | Flag            | Description                                 |
 | --------------- | ------------------------------------------- |
 | `--everything`  | Always stop containers and re-run rsync     |
-| `--dump-only`   | Only create SQL dumps, skip file backups    |
+| `--dump-only-sql`| Skip file backups only for DB volumes when dumps succeed; non-DB volumes are still backed up; fallback to files if no dump.    |
 | `--shutdown`    | Do not restart containers after backup      |
 | `--backups-dir` | Backup root directory (default: `/Backups`) |
 | `--repo-name`   | Backup namespace under machine hash         |