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/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