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>

CLAUDE.md

@@ -32,6 +32,7 @@ Internet ── host nginx (TLS, LE certs, HSTS) ──► caddy (127.0.0.1:8880
    auth.${BASE_DOMAIN}   ─► drasl        (Yggdrasil API + web UI)
    avatar.${BASE_DOMAIN} ─► nmsr         (skin/avatar renderer, Drasl-backed)
    status.${BASE_DOMAIN} ─► uptime-kuma  (status page + monitors)
+   files.${BASE_DOMAIN}  ─► filestash    (player file share, ONE shared login, writable)
    distribution.${BASE}  ─► static site from ANOTHER repo (DISTRIBUTION_WEB_ROOT)
 
 minecraft :25565 (+ 24454/udp Simple Voice Chat)
@@ -105,6 +106,17 @@ automatically; cosmetic-client mods that declare `BOTH` stay `both` until you
 hand-edit `mods-sides.json` to `client`. No Modrinth/packwiz, no network at
 classify/sync time. See `plan/04-mods.md`.
 
+### Filestash (files.) — three traps
+Full list in `plan/20-files.md`; the ones that cost real time:
+- The `local` backend is **admin-gated**: without `LOCAL_BACKEND_SECRET` in the
+  connection params, every login fails with `backend error - Not Allowed` (looks
+  like bad credentials, isn't).
+- `FILES_PASSWORD_HASH` must be **`$6$`** (`openssl passwd -6`). A `$2y$` bcrypt
+  from `htpasswd -B` fails every login silently — the plugin only matches `$2a$`.
+- `config.json` is rendered + mounted `:ro` (the admin console can't save, on
+  purpose). Single-file bind mount ⇒ **`--force-recreate` after re-rendering**,
+  never `restart`.
+
 ### Memory sizing
 ~50–100 mods, 8 players, 1.21.1: `INIT_MEMORY: 4G`, `MAX_MEMORY: 10G`,
 `USE_AIKAR_FLAGS: TRUE`.