docs(mqtt-auth): record verified VPS broker setup and ACL isolation test

Checked the live broker on 2026-09-30:
- mosquitto.conf sets no files-backend ACL path. A live test (device A
  subscribing to device B's topics) was denied while A's own topics were
  delivered, so the files backend does not grant-all and every ACL check
  reaches the Console. The missing ACL file is therefore harmless.
- Recorded the container/image, config and passwd locations, which lines
  already match the new code, and the one change still pending
  (auth/acl cache 300s -> 60s before app users go live).
- Added the copy-paste isolation test so it can be re-run after any broker
  config change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/mqtt-app-user-auth.md

@@ -111,14 +111,36 @@ When those lines stop appearing, set `MQTT_ALLOW_LEGACY_PASSWORD=false`.
        auth_opt_acl_jitter_seconds 10                 # r
        allow_anonymous false                          # R
 
-4. **Check the files ACL file** (`auth_opt_files_acl_path`). go-auth allows a request if
-   *any* backend allows it, so general rules in that file apply to app users too and
-   bypass the per-device ACL. Only lines under a `user admin` / `user bonamin` /
-   `user NodeRED` block are safe. A `pattern ...` line, or a `topic ...` line placed
-   before the first `user` line, applies to every user. Such lines must not cover
-   `vesper/...`.
+4. **Files-backend ACL: verified OK (2026-09-30).** The VPS config sets no
+   `auth_opt_files_acl_path` (nor `acl_file`). A live test confirmed the files backend
+   does **not** grant ACLs to everyone, so every ACL check reaches the Console. If an
+   ACL file is ever added, only lines under `user admin` / `user bonamin` /
+   `user NodeRED` are safe. A `pattern` line, or a `topic` line placed before the first
+   `user` line, applies to every user (app users included) and must not cover `vesper/...`.
+   Re-run the isolation test after any broker config change (on the VPS host):
+
+       A=PV25L22BP01R01; B=PV26B02BP01R01
+       PW=$(docker exec bellsystems-backend python -c "from mqtt.auth import _derive_password; print(_derive_password('$A'))")
+       # own topics -> should print messages
+       docker exec mosquitto mosquitto_sub -h localhost -i acl-test-own -u "$A" -P "$PW" -t "vesper/$A/#" -v -W 5
+       # foreign topics -> must say "All subscription requests were denied."
+       docker exec mosquitto mosquitto_sub -h localhost -i acl-test-foreign -u "$A" -P "$PW" -t "vesper/$B/#" -v -W 5
+
 5. **Enable a TLS listener** in Mosquitto for the app (certificate on the VPS).
 
+### Current VPS broker setup (as of 2026-09-30)
+
+- Container `mosquitto`, image `iegomez/mosquitto-go-auth:latest`.
+- Config: host `/home/bellsystems/stacks/mosquitto/config/mosquitto.conf`, mounted at
+  `/etc/mosquitto/mosquitto.conf` (Docker images usually use `/mosquitto/config`; this
+  one does not). Passwd file: `/home/bellsystems/stacks/mosquitto/passwd`.
+- Already set correctly: `backends files,http`, `http_host 172.20.0.1` (Docker bridge),
+  `http_port 8000`, both URIs, `http_method post`, `params_mode form`,
+  `response_mode status`, `cache true`, `cache_reset true`.
+- Still to change: `auth_cache_seconds` and `acl_cache_seconds` are **300**. Lower
+  both to 60 before app users go live, or a block/unassign can take up to 5 minutes
+  to apply.
+
 ## Known gaps / decisions
 
 - **FlutterFlow writes to `user_list`.** If the app changes `user_list` directly in