Add pl8list infinite-scroll app with pl8list sign-in

Endless TikTok-style feed over pl8list's Remix loaders. A guest crawler
builds a shared catalogue; a per-user ranker learns from likes, full
listens and skips. Users sign in with pl8list's email code; only a hash
of the local session token is stored. Ships as a hardened Docker image
with a Traefik override for pl8.narl.io.

.env.example

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+# Optional overrides. Defaults work in Docker.
+# HOST=127.0.0.1
+# PORT=8790
+# PL8_DB=/data/pl8list.db
+# Set by compose.override.yaml when behind Traefik:
+# TRUST_PROXY=1