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

templates/target_edit.html

@@ -67,6 +67,16 @@
                 </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>
+
+            <div class="form-group">
+                <label for="rotation" class="label">Archive Rotation</label>
+                <select id="rotation" name="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. A change applies from the next event; existing files keep their names.</p>
+            </div>
             {{end}}
 
             {{if or (eq .Target.Type "http") (eq .Target.Type "slack")}}