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: if one already has the
new name, the edit is refused. 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

internal/resetpw/resetpw_test.go

@@ -131,9 +131,15 @@ type noopNotifier struct{}
 
 func (n *noopNotifier) Notify([]delivery.Task) {}
 
-type noopEvictor struct{}
+type noopArchives struct{}
 
-func (n *noopEvictor) EvictWebhook(string) {}
+func (n *noopArchives) EvictWebhook(string) {}
+
+func (n *noopArchives) EvictTarget(string) {}
+
+func (n *noopArchives) Rename(_, _, _ string) error {
+	return nil
+}
 
 // newServerApp starts the real login path against dir: the handlers,
 // the middleware that bounds password verification, the session store
@@ -162,7 +168,7 @@ func newServerApp(
 			healthcheck.New,
 			session.New,
 			func() delivery.Notifier { return &noopNotifier{} },
-			func() delivery.WebhookEvictor { return &noopEvictor{} },
+			func() delivery.Archives { return &noopArchives{} },
 			middleware.New,
 			delivery.NewGuard,
 			handlers.New,