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

tests/e2e/helpers.py

@@ -166,7 +166,7 @@ def backup_run(
     database_containers: list[str],
     images_no_stop_required: list[str],
     images_no_backup_required: list[str] | None = None,
-    dump_only: bool = False,
+    dump_only_sql: bool = False,
 ) -> None:
     cmd = [
         "baudolo",
@@ -187,8 +187,8 @@ def backup_run(
     ]
     if images_no_backup_required:
         cmd += ["--images-no-backup-required", *images_no_backup_required]
-    if dump_only:
-        cmd += ["--dump-only"]
+    if dump_only_sql:
+        cmd += ["--dump-only-sql"]
 
     try:
         run(cmd, capture=True, check=True)