feat(backup): capture volumes from a filesystem snapshot

Backing up a live volume with rsync copies a moving target: a database
written to mid-copy lands on disk in a state no engine ever committed.
Stopping the container avoids that at the cost of downtime.

A snapshot removes both. `--snapshot {btrfs,zfs}` with `--snapshot-subject`
freezes the docker root once per run, and every volume copy is then read
from that frozen tree while the containers keep serving. A restore of such
a copy is an ordinary crash recovery, which every supported engine performs
on its own at startup.

An unsupported filesystem or an unknown snapshot kind fails loudly rather
than degrading to a live copy, since a silent fallback would return exactly
the torn backup the mode exists to prevent. `--shutdown` is rejected
alongside `--snapshot` instead of being ignored: under a snapshot no
container is ever stopped, so accepting the flag would promise downtime
semantics the run does not deliver.

Copies out of a snapshot skip rsync's --checksum verification. The source
is immutable for the lifetime of the copy, so size-and-mtime cannot race,
and dropping the second full read roughly halves the I/O per volume.

backup/app.py grew past what one module could carry and is split into
layout, policy and dumps along the lines it already had internally.

Tests: unit coverage for the new snapshot, layout, policy, volume and cli
units; e2e cases drive real btrfs, zfs and ext4 filesystems on loop devices
in a privileged container, including a MariaDB that is written to across
the snapshot and must recover from the restored copy without losing a
committed row. CI installs zfs and sets E2E_REQUIRE_FILESYSTEMS so a
missing kernel module fails the build instead of silently skipping a
filesystem.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/baudolo/backup/dumps.py

@@ -0,0 +1,98 @@
+"""Database dumps taken before a volume's files are copied."""
+
+from __future__ import annotations
+
+import sys
+
+import pandas
+from pandas.errors import EmptyDataError
+
+from .db import backup_database
+from .docker import has_image
+
+
+def backup_mariadb_or_postgres(
+    *,
+    container: str,
+    volume_dir: str,
+    databases_df: "pandas.DataFrame",
+    database_containers: list[str],
+) -> tuple[bool, bool]:
+    """
+    Returns (is_db_container, dumped_any)
+    """
+    for img in ["mariadb", "postgres"]:
+        if has_image(container, img):
+            dumped = backup_database(
+                container=container,
+                volume_dir=volume_dir,
+                db_type=img,
+                databases_df=databases_df,
+                database_containers=database_containers,
+            )
+            return True, dumped
+    return False, False
+
+
+def _empty_databases_df() -> "pandas.DataFrame":
+    """
+    Create an empty DataFrame with the expected schema for databases.csv.
+
+    This allows the backup to continue without DB dumps when the CSV is missing
+    or empty (pandas EmptyDataError).
+    """
+    return pandas.DataFrame(columns=["instance", "database", "username", "password"])
+
+
+def load_databases_df(csv_path: str) -> "pandas.DataFrame":
+    """
+    Load databases.csv robustly.
+
+    - Missing file     -> warn, continue with empty df
+    - Empty file       -> warn, continue with empty df
+    - Valid CSV        -> return dataframe
+    """
+    try:
+        return pandas.read_csv(csv_path, sep=";", keep_default_na=False, dtype=str)
+    except FileNotFoundError:
+        print(
+            f"WARNING: databases.csv not found: {csv_path}. Continuing without database dumps.",
+            file=sys.stderr,
+            flush=True,
+        )
+        return _empty_databases_df()
+    except EmptyDataError:
+        print(
+            f"WARNING: databases.csv exists but is empty: {csv_path}. Continuing without database dumps.",
+            file=sys.stderr,
+            flush=True,
+        )
+        return _empty_databases_df()
+
+
+def backup_dumps_for_volume(
+    *,
+    containers: list[str],
+    vol_dir: str,
+    databases_df: "pandas.DataFrame",
+    database_containers: list[str],
+) -> tuple[bool, bool]:
+    """
+    Returns (found_db_container, dumped_any)
+    """
+    found_db = False
+    dumped_any = False
+
+    for c in containers:
+        is_db, dumped = backup_mariadb_or_postgres(
+            container=c,
+            volume_dir=vol_dir,
+            databases_df=databases_df,
+            database_containers=database_containers,
+        )
+        if is_db:
+            found_db = True
+        if dumped:
+            dumped_any = True
+
+    return found_db, dumped_any