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/handlers/target_paused_test.go

@@ -107,7 +107,8 @@ func TestPausedTarget_ShownUntilBreakerCloses(t *testing.T) {
 	assert.Contains(t, list, "t-http http Active Edit Deactivate Delete "+
 		"Deliveries Paused: held while one delivery tests whether the "+
 		"target has recovered")
-	assert.NotContains(t, list, "UTC")
+	// Not the whole list: the add target form above the rows says UTC.
+	assert.NotContains(t, targetRow(list, "t-http", "t-log"), "UTC")
 
 	assertRetryingNotWaiting(t, h, sess, wh.ID, retrying, backedOff)
 
@@ -119,6 +120,15 @@ func TestPausedTarget_ShownUntilBreakerCloses(t *testing.T) {
 	assertRetryingNotWaiting(t, h, sess, wh.ID, retrying, backedOff)
 }
 
+// targetRow returns the row of the target named name in a targetList:
+// from its name to the name of the target listed after it, next.
+func targetRow(list, name, next string) string {
+	_, row, _ := strings.Cut(list, name+" ")
+	row, _, _ = strings.Cut(row, next+" ")
+
+	return row
+}
+
 // assertRetryingNotWaiting checks that the event log and each event's
 // page show the http target's delivery of the event as retrying, and
 // none of them as waiting.