docmost: remove the client-written X-Forwarded-For chain on its router (R-753)

Fastify trustProxy true — leftmost XFF; its login limit (10/min) is keyed by IP only. Once traefik trusts the tunnel's fixed address (controller v0.286.0), the leftmost entry is what a
stranger writes; with the chain removed the app reads traefik's X-Real-Ip or its peer, as before — never forgeable.
Measured on 9202: a router with this middleware receives no X-Forwarded-For (felhom.eu audits/visitors-2026-10-01/A/P1).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

templates/docmost/docker-compose.yml

@@ -49,6 +49,10 @@ services:
     labels:
       - "traefik.enable=true"
       - "traefik.http.routers.docmost.rule=Host(`${SUBDOMAIN}.${DOMAIN}`)"
+      # R-753: this app reads the LEFTMOST X-Forwarded-For entry, which a stranger writes once traefik keeps the
+      # tunnel's chain — so its chain is removed here (it then reads traefik's X-Real-Ip or peer; never forgeable).
+      - "traefik.http.middlewares.docmost-xff.headers.customrequestheaders.X-Forwarded-For="
+      - "traefik.http.routers.docmost.middlewares=docmost-xff"
       - "traefik.http.routers.docmost.entrypoints=websecure"
       - "traefik.http.routers.docmost.tls=true"
       - "traefik.http.routers.docmost.tls.certresolver=letsencrypt"