Approximate the amount of space freed during backup rotation

The current output is somewhat hard to read because it displays the
size in bytes.

Before:

```
Sep 17 03:00:02 xsnippet python3[2230826]: Used space: 30 files, 133721164 bytes
Sep 17 03:00:02 xsnippet python3[2230826]: Freed space: 1 files, 4455144 bytes
```

After:

```
Sep 17 03:00:02 xsnippet python3[2230826]: Used space: 30 files, 127.5 MiB
Sep 17 03:00:02 xsnippet python3[2230826]: Freed space: 1 files, 4.2 MiB
```

Author: malor

roles/postgres/files/rotate.py

@@ -97,8 +97,26 @@ def cleanup(to_keep: list[Backup], to_remove: list[Backup], *, dry_run: bool = T
         for backup in to_remove:
             backup.path.unlink()
 
-    print(f"Used space: {len(to_keep)} files, {used_space} bytes")
-    print(f"Freed space: {len(to_remove)} files, {freed_space} bytes")
+    print(f"Used space: {len(to_keep)} files, {approximate_size(used_space)}")
+    print(f"Freed space: {len(to_remove)} files, {approximate_size(freed_space)}")
+
+
+def approximate_size(size: int) -> str:
+    """Convert size in bytes to a human-readable form."""
+
+    units = {
+        2**10: "KiB",
+        2**20: "MiB",
+        2**30: "GiB",
+        2**40: "TiB",
+        2**50: "PiB",
+    }
+
+    for multiplier, name in reversed(units.items()):
+        if size >= multiplier:
+            return "{:.1f} {}".format(size / multiplier, name)
+
+    return "{} B".format(size)
 
 
 def main():

roles/postgres/files/test_rotate.py

@@ -3,6 +3,8 @@
 
 import pytest
 
+import rotate
+
 
 EXPECTED_FILES = [
     "2024-06-27.tar.gz",
@@ -123,3 +125,26 @@ def test_keep_pattern_does_not_match_anything(backups):
         ]
     )
     assert sorted(item.name for item in backups.glob("*")) == EXPECTED_FILES
+
+
+@pytest.mark.parametrize(
+    "size,expected",
+    [
+        (0, "0 B"),
+        (1, "1 B"),
+        (1023, "1023 B"),
+        (1024, "1.0 KiB"),
+        (1025, "1.0 KiB"),
+        (1024 * 1024, "1.0 MiB"),
+        (5 * 1024 * 1024, "5.0 MiB"),
+        (1024 * 1024 * 1024, "1.0 GiB"),
+        (10 * 1024 * 1024 * 1024, "10.0 GiB"),
+        (1024 * 1024 * 1024 * 1024, "1.0 TiB"),
+        (13.5 * 1024 * 1024 * 1024 * 1024, "13.5 TiB"),
+        (1024 * 1024 * 1024 * 1024 * 1024, "1.0 PiB"),
+        (145.7 * 1024 * 1024 * 1024 * 1024 * 1024, "145.7 PiB"),
+        (1024 * 1024 * 1024 * 1024 * 1024 * 1024, "1024.0 PiB"),
+    ],
+)
+def test_approximate_size(size, expected):
+    assert rotate.approximate_size(size) == expected