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/Header.tsx

@@ -33,7 +33,7 @@ export default function Header({
         className="text-xl font-bold tracking-tight select-none"
         title="Feed"
       >
-        Sub<span className="text-accent">feed</span>
+        Sift<span className="text-accent">lode</span>
       </button>
 
       <SyncStatus />