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: each name is lowercased, keeps ASCII letters and digits, turns
every other run of characters into one dash, and is cut to 40
characters.

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. 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(