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_rotation.go

@@ -0,0 +1,42 @@
+package handlers
+
+const (
+	// archiveRotationNone is the archive rotation that keeps a
+	// database target's archive in one file. A stored empty rotation
+	// means the same.
+	archiveRotationNone = "none"
+
+	// tmplKeyArchiveRotationChoices is the template data key for the
+	// entries of a page's archive rotation select.
+	tmplKeyArchiveRotationChoices = "ArchiveRotationChoices"
+)
+
+// archiveRotationChoices lists the archive rotations offered by the
+// new webhook page, the add target form and the target edit form.
+func archiveRotationChoices() []archiveChoice {
+	return []archiveChoice{
+		{Value: archiveRotationNone, Label: archiveRotationNone},
+		{Value: "monthly", Label: "monthly"},
+		{Value: "daily", Label: "daily"},
+		{Value: "hourly", Label: "hourly"},
+	}
+}
+
+// archiveRotationOptions returns the choices with rotation selected;
+// an empty rotation, or one that is not a choice, selects none. A
+// stored rotation is always a choice: the forms refuse any other.
+func archiveRotationOptions(rotation string) []archiveChoice {
+	options := archiveRotationChoices()
+
+	for i := range options {
+		if options[i].Value == rotation {
+			options[i].Selected = true
+
+			return options
+		}
+	}
+
+	options[0].Selected = true
+
+	return options
+}