Evict archive writers on deletion and sweep idle archives (closes #89) (#95)

Per-webhook archive writers are now evicted when the webhook or its last
database target is deleted, and a background sweeper prunes expired rows from
idle archives that no longer receive writes. Archive files themselves are never
deleted.

cmd/webhooker/main.go

@@ -40,9 +40,15 @@ func main() {
 			handlers.New,
 			middleware.New,
 			delivery.New,
+			delivery.NewArchiveSweeper,
 			// 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
+			},
 			server.New,
 		),
 		fx.Invoke(
@@ -50,6 +56,7 @@ func main() {
 				*server.Server,
 				*delivery.Engine,
 				*database.RetentionReaper,
+				*delivery.ArchiveSweeper,
 			) {
 			},
 		),