Rotate a database target's archive monthly, daily or hourly (closes #379)

A database target's rotation (none, monthly, daily or hourly) puts the
UTC period of each event's receive time in its archive file name, so
each file holds exactly its period's events. It is on the new webhook
page, the add target form and the target edit form, and shown in the
target list.

Renames move every one of a target's files and move them back if one
fails. The sweep prunes every file, one at a time under the target's
lock, and deletes a rotated file it leaves empty. Download lists the
files, then opens one at a time, oldest first, finding each again under
the target's current names, and gives each row its period. The target
list names the current file and totals the size of all of them.

Model: opus-5-5

internal/delivery/target_database_test.go

@@ -218,6 +218,7 @@ func TestStatArchive(t *testing.T) {
 
 	got, err := delivery.StatArchive(path)
 	require.NoError(t, err)
+	assert.Equal(t, 1, got.Files)
 	assert.Equal(t, file.Size()+wal.Size(), got.Size)
 	assert.True(t, written.Equal(got.Written), got.Written)
 
@@ -720,3 +721,41 @@ func TestArchiveWriter_RenameMovesBackOnFailure(t *testing.T) {
 	assert.NoFileExists(t, filepath.Join(dir, newName))
 	assert.Equal(t, oldPath, w.Path())
 }
+
+// TestArchiveWriter_RenameMovesBackEveryFile renames a target with a
+// file without a period and a file for a month, and proves that when
+// the month's file fails to move, the file already moved is moved back:
+// both files are under the old name with their rows, and nothing is
+// under the new name. The new name is 251 bytes, so the file without a
+// period, with its -wal and -shm, can take it, but the month's file,
+// eight bytes longer, cannot.
+func TestArchiveWriter_RenameMovesBackEveryFile(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+	oldPath := filepath.Join(dir, "archive-old.db")
+	monthPath := filepath.Join(dir, "archive-old-2026-03.db")
+	w := delivery.NewExportArchiveWriter(oldPath, archiveTestLogger(), 0)
+
+	require.NoError(t, w.WritePeriod(
+		delivery.ExportArchivedEvent{EventID: "in-none"}, 0, "",
+	))
+	require.NoError(t, w.WritePeriod(
+		delivery.ExportArchivedEvent{EventID: "in-month"}, 0, "2026-03",
+	))
+
+	require.Error(t, w.Rename(strings.Repeat("a", 248)+".db"))
+
+	assert.Equal(t, []string{"in-none"}, archivedEventIDs(t, oldPath))
+	assert.Equal(t, []string{"in-month"}, archivedEventIDs(t, monthPath))
+
+	entries, err := os.ReadDir(dir)
+	require.NoError(t, err)
+
+	for _, entry := range entries {
+		assert.True(t, strings.HasPrefix(entry.Name(), "archive-old"),
+			"%s is not under the old name", entry.Name())
+	}
+
+	assert.Equal(t, oldPath, w.Path())
+}