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

src/baudolo/backup/cli.py

@@ -68,10 +68,15 @@ def parse_args() -> argparse.Namespace:
         action="store_true",
         help="Do not restart containers after backup",
     )
+    
     p.add_argument(
-        "--dump-only",
+        "--dump-only-sql",
         action="store_true",
-        help="Only create DB dumps (skip ALL file rsync backups)",
+        help=(
+            "Create database dumps only for DB volumes. "
+            "File backups are skipped for DB volumes if a dump succeeds, "
+            "but non-DB volumes are still backed up. "
+            "If a DB dump cannot be produced, baudolo falls back to a file backup."
+        ),
     )
-
     return p.parse_args()