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

static/js/app.js

@@ -72,8 +72,8 @@ document.addEventListener("alpine:init", function () {
     // Something a click shows and hides: the mobile menu, an add form,
     // an entrypoint's edit form, an event in the event log or in the
     // recent events, a delivery's attempts, the new webhook page's
-    // archive pruning choice. It starts hidden, or shown when its
-    // element has the data-open attribute.
+    // archive pruning and rotation choices. It starts hidden, or shown
+    // when its element has the data-open attribute.
     window.Alpine.data("collapsible", function () {
         return {
             open: false,
@@ -140,6 +140,7 @@ document.addEventListener("alpine:init", function () {
             timeout: "",
             maxRetries: "",
             expiry: "",
+            rotation: "",
             init() {
                 const refused = this.$root.dataset;
 
@@ -151,6 +152,7 @@ document.addEventListener("alpine:init", function () {
                 this.timeout = refused.timeout;
                 this.maxRetries = refused.maxRetries;
                 this.expiry = refused.expiry;
+                this.rotation = refused.rotation;
             },
             add() {
                 this.choosing = true;
@@ -169,6 +171,7 @@ document.addEventListener("alpine:init", function () {
                 this.timeout = "";
                 this.maxRetries = "";
                 this.expiry = "";
+                this.rotation = "";
                 this.$refs.form.reset();
             },
             get filling() {