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_download.go

@@ -86,9 +86,10 @@ func (d downloadWriter) Write(b []byte) (int, error) {
 // written the response.
 //
 // It holds renameMu, which every archive rename runs under, while it
-// reads the stored names and opens the file, so the file it opens is
-// the one those names give. It lets go before the export is streamed:
-// once the file is open, a rename does not affect the export.
+// reads the stored names and opens the files, so the files it opens
+// are the ones those names give. It lets go before the export is
+// streamed: once the files are open, a rename does not affect the
+// export.
 func (h *Handlers) openTargetArchive(
 	ctx context.Context,
 	w http.ResponseWriter,