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, the sweep prunes every file
and deletes a rotated file it leaves empty, Download exports every file
oldest first with each row's period, and the target list names the
current file and totals the size of all of them.

Model: opus-5-5

internal/delivery/target_database_export_test.go

@@ -301,6 +301,56 @@ func TestArchiveExport_SurvivesRename(t *testing.T) {
 	)
 }
 
+// TestArchiveExport_EveryFileOldestFirst writes a row to a target's
+// file without a period and to its files for a month, an hour and a
+// day, and proves the export holds every row: the file without a
+// period first, then the others oldest period first, each row from a
+// file named for a period carrying that period. A file made after the
+// export opened is not in it.
+func TestArchiveExport_EveryFileOldestFirst(t *testing.T) {
+	t.Parallel()
+
+	path := filepath.Join(t.TempDir(), "archive-wh.db")
+	w := delivery.NewExportArchiveWriter(path, archiveTestLogger(), 0)
+
+	// Written in an order that is not the export's.
+	for _, period := range []string{nextDayPeriod, "", hourPeriod, "2026-03"} {
+		require.NoError(t, w.WritePeriod(
+			delivery.ExportArchivedEvent{EventID: "in-" + period}, 0, period,
+		))
+	}
+
+	export, err := delivery.OpenArchiveExport(
+		t.Context(), path, archiveTestLogger(),
+	)
+	require.NoError(t, err)
+
+	defer func() { require.NoError(t, export.Close()) }()
+
+	require.NoError(t, w.WritePeriod(
+		delivery.ExportArchivedEvent{EventID: "later"}, 0, "2026-03-06",
+	))
+
+	events := exportedEvents(t, writeExport(t, export))
+	ids := make([]string, 0, len(events))
+	periods := make([]any, 0, len(events))
+
+	for _, ev := range events {
+		ids = append(ids, fmt.Sprint(ev["event_id"]))
+		periods = append(periods, ev["period"])
+	}
+
+	assert.Equal(t,
+		[]string{"in-", "in-2026-03", "in-" + hourPeriod, "in-" + nextDayPeriod},
+		ids,
+	)
+	assert.Equal(t,
+		[]any{nil, "2026-03", hourPeriod, nextDayPeriod}, periods,
+	)
+	assert.NotContains(t, events[0], "period",
+		"a row from the file without a period has no period")
+}
+
 // heapPeak is an io.Writer that discards what it is given and records
 // the largest heap it saw at a write. It collects garbage before each
 // reading, so the heap it reads is what is still held.