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/legal/LegalLayout.tsx

@@ -24,7 +24,7 @@ export default function LegalLayout({
     <div className="min-h-screen bg-bg text-fg">
       <div className="mx-auto w-[min(92vw,720px)] py-12">
         <a href="/" className="inline-block text-2xl font-bold tracking-tight mb-8">
-          Sub<span className="text-accent">feed</span>
+          Sift<span className="text-accent">lode</span>
         </a>
         <div className="glass rounded-2xl p-8">
           <h1 className="text-2xl font-semibold">{title}</h1>