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/target_edit_test.go

@@ -591,7 +591,8 @@ func TestHandleTargetEditSubmit_RefusedFormComesBack(t *testing.T) {
 			"Invalid max retries",
 		},
 		{
-			database.TargetTypeDatabase, "name=edited&expiry=7d",
+			database.TargetTypeDatabase,
+			"name=edited&expiry=7d&rotation=daily",
 			"Invalid archive expiry",
 		},
 		{database.TargetTypeLog, "name=", "Name is required"},
@@ -613,15 +614,15 @@ func TestHandleTargetEditSubmit_RefusedFormComesBack(t *testing.T) {
 			page := w.Body.String()
 			assert.Contains(t, page, `class="alert-error">`+tc.reason)
 
-			// headers is the form's one textarea and expiry its one
-			// select; every other field is an input.
+			// headers is the form's one textarea, and expiry and
+			// rotation its selects; every other field is an input.
 			for field := range form {
 				shown := `name="` + field + `" value="` + form.Get(field) + `"`
 
 				switch field {
 				case "headers":
 					shown = ">" + form.Get(field) + "</textarea>"
-				case "expiry":
+				case "expiry", "rotation":
 					shown = `<option value="` + form.Get(field) + `" selected>`
 				}