Rotate a database target's archive monthly, daily or hourly (closes #379)

A database target's rotation setting (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 and both target forms, and shown in the target list. Renames move every one of a target's files and move them all back if one fails. The sweep prunes one file at a time under the target's lock and deletes a rotated file it leaves empty. Download opens one file at a time, oldest period first, finding each again under the target's current name. The target list names the current file and totals 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 {