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/handlers/archive_expiry_test.go

@@ -5,6 +5,7 @@ import (
 	"net/http/httptest"
 	"net/url"
 	"regexp"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -49,21 +50,39 @@ func expiryShown(
 }
 
 // expirySelected returns the target edit page and the expiries its
-// select starts on.
+// expiry select starts on.
 func expirySelected(
 	t *testing.T, env *sourceTestEnv, webhookID, targetID string,
 ) (string, []string) {
 	t.Helper()
 
+	page := targetEditPage(t, env, webhookID, targetID)
+
+	return page, selectedIn(page, "expiry")
+}
+
+// targetEditPage returns a target's edit page.
+func targetEditPage(
+	t *testing.T, env *sourceTestEnv, webhookID, targetID string,
+) string {
+	t.Helper()
+
 	w := serveTarget(
 		env, http.MethodGet,
 		"/hook/"+webhookID+"/targets/"+targetID+"/edit", nil,
 	)
 	require.Equal(t, http.StatusOK, w.Code)
 
-	page := w.Body.String()
+	return w.Body.String()
+}
 
-	return page, matched(`<option value="([^"]*)" selected>`, page)
+// selectedIn returns the values the select named name on page starts
+// on.
+func selectedIn(page, name string) []string {
+	_, rest, _ := strings.Cut(page, `<select id="`+name+`" name="`+name+`"`)
+	options, _, _ := strings.Cut(rest, "</select>")
+
+	return matched(`<option value="([^"]*)" selected>`, options)
 }
 
 // TestArchiveExpiryChoices adds a database target with each archive