Name each database target's archive for its webhook and target (closes #376)

Each database target now has its own archive file,
archive-WEBHOOKNAME-TARGETNAME-TARGETID.db, instead of one
archive-WEBHOOKID.db per webhook. delivery.ArchiveFileName builds the
name.

A change of webhook or target name renames its archive files under the
archive writer's lock, before the new name is saved, and back again if
the save fails. Webhook edits, target edits and target creation run one
at a time, so no two of them interleave. A rename never replaces a
file, and one that fails part way moves back what it moved. Deleting a
target evicts only that target's writer. Archive files are never
deleted, and nothing looks for files under the old name.

Model: opus-5-5

cmd/webhooker/main.go

@@ -192,11 +192,10 @@ func newApp() *fx.App {
 			// Wire *delivery.Engine as delivery.Notifier so the
 			// webhook handler can notify the engine of new deliveries.
 			func(e *delivery.Engine) delivery.Notifier { return e },
-			// Wire *delivery.Engine as delivery.WebhookEvictor so
-			// deleting a webhook releases its archive writer.
-			func(e *delivery.Engine) delivery.WebhookEvictor {
-				return e
-			},
+			// Wire *delivery.Engine as delivery.Archives so deleting
+			// or renaming a webhook or target reaches its archive
+			// files.
+			func(e *delivery.Engine) delivery.Archives { return e },
 			server.New,
 		),
 		fx.Invoke(