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

internal/delivery/engine.go

@@ -295,12 +295,13 @@ func (e *Engine) EvictTarget(targetID string) {
 	e.dbTarget.evict(targetID)
 }
 
-// Rename implements Archives. It renames a database target's
-// archive file to ArchiveFileName(webhookName, targetName,
-// targetID), under the lock the target's archive writes and the
-// idle sweep take. It never replaces a file: if one already has the
-// new name, the error is ErrArchiveNameTaken. The caller renames
-// before it saves the new name: see databaseTarget.rename.
+// Rename implements Archives. It renames every one of a database
+// target's archive files to ArchiveFileName(webhookName, targetName,
+// targetID), each keeping the period in its name, under the lock the
+// target's archive writes and the idle sweep take. It never replaces
+// a file: if one already has a new name, the error is
+// ErrArchiveNameTaken. The caller renames before it saves the new
+// name: see databaseTarget.rename.
 func (e *Engine) Rename(
 	targetID, webhookName, targetName string,
 ) error {