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

templates/sources_new.html

@@ -38,9 +38,9 @@
                 <p class="text-xs text-gray-500 mt-1">Optional. When filled in, the webhook is created with an HTTP target that delivers each event to this URL.</p>
             </div>
 
-            <!-- The checkbox shows the pruning choice while checked. With
-                 autocomplete="off", going back to the page does not
-                 check the box again with the choice hidden. -->
+            <!-- The checkbox shows the pruning and rotation choices while
+                 checked. With autocomplete="off", going back to the page
+                 does not check the box again with the choices hidden. -->
             <div class="form-group" x-data="collapsible"{{if .Form.Archive}} data-open{{end}}>
                 <label class="flex items-center gap-2 text-sm font-medium text-gray-700">
                     <input type="checkbox" name="archive" value="on"{{if .Form.Archive}} checked{{end}} autocomplete="off" @change="toggle" class="h-4 w-4">
@@ -55,6 +55,15 @@
                         {{end}}
                     </select>
                     <p class="text-xs text-gray-500 mt-1">Archived events older than this are deleted from the archive; never keeps them all.</p>
+                    <div class="mt-3">
+                        <label for="archive_rotation" class="label">Archive rotation</label>
+                        <select id="archive_rotation" name="archive_rotation" class="input">
+                            {{range .ArchiveRotationChoices}}
+                            <option value="{{.Value}}"{{if .Selected}} selected{{end}}>{{.Label}}</option>
+                            {{end}}
+                        </select>
+                        <p class="text-xs text-gray-500 mt-1">Starts a new archive file each month, day or hour (UTC), named for it; none keeps one file.</p>
+                    </div>
                 </div>
             </div>