feat(files): add Filestash player file share at files.${BASE_DOMAIN}

One shared login (htpasswd) for all players, writable. Config is rendered
from a template and mounted read-only, so git stays authoritative — the
admin console loads but cannot save, by design.

Filestash's OIDC/SAML middlewares are enterprise-only, so Keycloak SSO is
out; FILES_AUTH keeps htpasswd/passthrough switchable for later. Auth-off
(passthrough) is only valid once the host is off the public internet —
render-config.sh warns loudly when rendering it.

Three traps, all found by testing against the pinned image:
- the `local` backend is admin-gated: without LOCAL_BACKEND_SECRET in the
  connection params every login fails with "backend error - Not Allowed"
- the htpasswd plugin only accepts $2a$ bcrypt, so a $2y$ hash from
  `htpasswd -B` fails every login silently — use `openssl passwd -6`.
  render-config.sh rejects the wrong format rather than shipping it
- APPLICATION_URL/ADMIN_PASSWORD env make filestash rewrite config.json at
  boot, which fails on the :ro mount — both live in the rendered config

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

.gitignore

@@ -6,6 +6,9 @@ runtime/authlib-injector.jar
 # rendered configs (from tooling/render-config.sh)
 drasl/config/config.toml
 nmsr/config.toml
+# holds the admin bcrypt hash, the shared player hash, the session key and the
+# local-backend secret — only the .tmpl is tracked
+filestash/config.json
 
 # server mod subset synced from the distribution repo by
 # tooling/sync-server-mods.sh (source of truth is mods-sides.json + $DIST jars)