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/app.py

@@ -149,17 +149,20 @@ def main() -> int:
             database_containers=args.database_containers,
         )
 
-        # dump-only logic:
-        if args.dump_only:
-            if found_db and not dumped_any:
-                print(
-                    f"WARNING: dump-only requested but no DB dump was produced for DB volume '{volume_name}'. Falling back to file backup.",
-                    flush=True,
-                )
-                # continue to file backup below
-            else:
-                # keep old behavior: skip file backups
-                continue
+        # dump-only-sql logic:
+        if args.dump_only_sql:
+            if found_db:
+                if not dumped_any:
+                    print(
+                        f"WARNING: dump-only-sql requested but no DB dump was produced for DB volume '{volume_name}'. Falling back to file backup.",
+                        flush=True,
+                    )
+                    # fall through to file backup below
+                else:
+                    # DB volume successfully dumped -> skip file backup
+                    continue
+            # Non-DB volume -> always do file backup (fall through)
+
 
 
         # skip file backup if all linked containers are ignored

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()