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>

filestash/config.json.tmpl

@@ -1,7 +1,7 @@
 {
     "general": {
         "name": "Ulicraft Files",
-        "host": "files.${BASE_DOMAIN}",
+        "host": "upload.${BASE_DOMAIN}",
         "force_ssl": true,
         "secret_key": "${FILES_SECRET_KEY}",
         "editor": "base",