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

A database target's rotation setting (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 and both target forms, and shown in the target list. Renames move every one of a target's files and move them all back if one fails. The sweep prunes one file at a time under the target's lock and deletes a rotated file it leaves empty. Download opens one file at a time, oldest period first, finding each again under the target's current name. The target list names the current file and totals 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())
+}