build(lint): gate make test on a clean ruff run

ruff was never wired into this repository: no target, no CI step, no pin.
It reported 45 findings across sources and tests, so nothing enforced
what the codebase already mostly followed.

Adds `make ruff` (check + format --check), `make ruff-fix`, and `make
lint` as its alias, and makes `make test` run lint as a fourth parallel
spur. The CI workflow calls `make test`, so it is covered there too. The
linter is pinned in a `lint` extra: a ruff minor bump changes which rules
fire, and with the suite gating on a clean run an unpinned linter would
fail it on an unrelated day.

The 45 findings are fixed rather than configured away. Three needed a
decision instead of the mechanical fix:

- The generation timestamp keeps its local wall clock (DTZ005 waived).
  Generations sort by that name, and UTC would order new ones before the
  existing ones wherever the offset is positive - "newest generation" is
  what every restore path selects on.
- The per-volume `copy` closure now binds volume_name and vol_dir as
  default arguments (B023). It only worked because it is called inside
  the same iteration.
- The two CLI top-level handlers keep their blind except (BLE001
  waived): turning any failure into exit 1 is what a CLI boundary is
  for. The two in run.py did not need it and were narrowed to what they
  actually catch.

Also drops the comments that restate the code: the section banners in
restore/__main__.py, the filename repeated as line 1 of nine test files,
step narration above the statement it narrates, and a block in app.py
documenting parameters that had moved to another module. What names a
trip-wire stays - the snapshot destination rule, the mysql-binary
absence in MariaDB 11 images, the session-scoped FOREIGN_KEY_CHECKS, the
spooled temp file for multi-GB dumps, and the negative control that
loses its discriminating power if it ever passes.

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

tests/unit/backup/test_snapshot.py

@@ -61,9 +61,11 @@ class TestBtrfs(unittest.TestCase):
 
     def test_it_removes_the_snapshot_even_when_the_body_raises(self) -> None:
         run = Runner()
-        with self.assertRaises(ZeroDivisionError):
-            with volume_snapshot("btrfs", "/var/lib/docker", "20260731", run=run):
-                raise ZeroDivisionError
+        with (
+            self.assertRaises(ZeroDivisionError),
+            volume_snapshot("btrfs", "/var/lib/docker", "20260731", run=run),
+        ):
+            raise ZeroDivisionError
         self.assertTrue(run.calls[-1].startswith("btrfs subvolume delete"))
 
 
@@ -93,26 +95,30 @@ class TestZfs(unittest.TestCase):
 
     def test_an_unmounted_dataset_is_an_error(self) -> None:
         run = Runner({"zfs list": [""]})
-        with self.assertRaises(SnapshotError):
-            with volume_snapshot("zfs", "/var/lib/docker", "20260731", run=run):
-                pass
+        with (
+            self.assertRaises(SnapshotError),
+            volume_snapshot("zfs", "/var/lib/docker", "20260731", run=run),
+        ):
+            pass
 
 
 class TestRejections(unittest.TestCase):
     def test_an_unknown_kind_is_rejected(self) -> None:
         run = Runner()
-        with self.assertRaises(SnapshotError):
-            with volume_snapshot("ext4", "/var/lib/docker", "20260731", run=run):
-                pass
+        with (
+            self.assertRaises(SnapshotError),
+            volume_snapshot("ext4", "/var/lib/docker", "20260731", run=run),
+        ):
+            pass
         self.assertEqual(run.calls, [])
 
     def test_a_path_outside_the_subject_is_rejected(self) -> None:
         run = Runner()
-        with volume_snapshot(
-            "btrfs", "/var/lib/docker", "20260731", run=run
-        ) as resolve:
-            with self.assertRaises(SnapshotError):
-                resolve("/etc/passwd")
+        with (
+            volume_snapshot("btrfs", "/var/lib/docker", "20260731", run=run) as resolve,
+            self.assertRaises(SnapshotError),
+        ):
+            resolve("/etc/passwd")
 
     def test_the_subject_itself_resolves_to_the_snapshot_root(self) -> None:
         run = Runner()
@@ -137,9 +143,11 @@ class TestRemovalFailure(unittest.TestCase):
             pass
 
     def test_a_failed_removal_does_not_mask_the_body(self) -> None:
-        with self.assertRaises(ZeroDivisionError):
-            with volume_snapshot("btrfs", "/var/lib/docker", "20260731", run=Busy()):
-                raise ZeroDivisionError
+        with (
+            self.assertRaises(ZeroDivisionError),
+            volume_snapshot("btrfs", "/var/lib/docker", "20260731", run=Busy()),
+        ):
+            raise ZeroDivisionError
 
 
 if __name__ == "__main__":