feat(files): move filestash to upload., serve files. as read-only static browse

Filestash cannot be served under a subpath (its SPA hardcodes /api, /assets,
/login at root), so it moves to its own subdomain upload.${BASE_DOMAIN}. The
apex-level ask was a static, no-auth file browser at files.${BASE_DOMAIN}: caddy
now file_server browse's the same ${FILES_DATA_DIR} share read-only there.

- filestash general.host -> upload.; nginx upload. vhost keeps the 2g body +
  upload streaming + /admin throttle; new files. vhost is a plain static proxy.
- caddy: 60-files.caddy split into 60-upload.caddy (filestash) + 62-files.caddy
  (file_server browse of /srv/files); caddy mounts FILES_DATA_DIR :ro.
- LE_SUBDOMAINS += upload (both upload. and files. are vhosts needing certs).

Security: files. is unconditionally public read (no auth), independent of
FILES_AUTH. The htpasswd login only gates writes via upload. Documented.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.env.example

@@ -49,7 +49,8 @@ CADDY_HTTP_BIND=127.0.0.1
 # static site — it lives in ANOTHER repo. Bind-mounted read-only into caddy.
 DISTRIBUTION_WEB_ROOT=/home/oier/projects/mc-mods/ulicraft-group/ulicraft-distribution/dist
 
-# ── files.${BASE_DOMAIN} — Filestash player file share ────────────────
+# ── upload.${BASE_DOMAIN} — Filestash player file share (write) ───────
+# (files.${BASE_DOMAIN} = read-only, no-auth caddy browse of the same dir)
 # Screenshots / schematics / maps. ONE shared login for all players, writable.
 # Full design + gotchas: plan/20-files.md.
 
@@ -126,7 +127,7 @@ GENERATORS_DATA_DIR=./generators-data
 # Only needed to issue real TLS certs. DNS-01 needs no inbound ports.
 LE_EMAIL=you@example.com
 # space-separated subdomains; apex ${BASE_DOMAIN} is always included
-LE_SUBDOMAINS=auth distribution www avatar status files generators
+LE_SUBDOMAINS=auth distribution www avatar status upload files generators
 # OVH API creds (DNS zone write). Create at https://eu.api.ovh.com/createToken/
 # OVH_CK can be blank on first run — acme.sh prints an auth URL, then paste it.
 OVH_END_POINT=ovh-eu