chore: rebrand Subfeed -> Siftlode

Rename all user-facing references (UI wordmark Sift+lode, titles, app name,
legal pages, onboarding wizard, emails, README/docs) and infra paths
(/srv/subfeed -> /srv/siftlode, image tag, deploy script, backup filenames).

Internal identifiers kept on purpose: Postgres user/db "subfeed", logger
namespace, localStorage keys, and the subfeed_pgdata volume (renaming would
orphan the migrated production data).

frontend/src/components/Login.tsx

@@ -32,7 +32,7 @@ export default function Login() {
     <div className="min-h-screen grid place-items-center bg-bg text-fg">
       <div className="glass w-[min(92vw,420px)] rounded-2xl p-10 text-center">
         <div className="text-3xl font-bold tracking-tight">
-          Sub<span className="text-accent">feed</span>
+          Sift<span className="text-accent">lode</span>
         </div>
         <p className="text-muted my-5 leading-relaxed">
           A self-hosted, filterable feed of your YouTube subscriptions — sort, tag, and