feat(6838): read the client address and HTTPS from the trusted proxy hop

grimoire/proxy.py is the one reader of X-Forwarded-For and X-Forwarded-Proto.
It takes the entry TRUSTED_PROXY_HOPS in from the right (default 1; a shorter
header reads its leftmost entry; 0 trusts no proxy), so an entry a client puts
left of the proxy's is never believed. Before this, with TRUST_PROXY_HEADERS
on, the leftmost X-Forwarded-For entry keyed the login, register, forgot and
reset rate limits and the audit log, and a client could choose it.

The session cookie is Secure, and HSTS (max-age=31536000) is sent, exactly
when the request is HTTPS by the trusted proxy's word. Plain HTTP is
untouched and nothing redirects. SECURE_COOKIES is gone (a startup warning says
so when it is still set); TRUST_PROXY_HEADERS=false still maps to 0 hops, with
a warning to move to TRUSTED_PROXY_HOPS. Admin audit lines now use the same
client address instead of the socket peer.

The placeholder-key refusal keyed on SECURE_COOKIES, so it now refuses either
placeholder key outright; the instance makes its own key when none is set.

Family idea #5004 (IsHTTPS, trusted-hop X-Forwarded-Proto), canon v2.

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

docker-compose.prod.yml

@@ -8,8 +8,9 @@ services:
       # a Docker secret and sets SECRET_KEY_FILE to its path).
       SECRET_KEY: "${SECRET_KEY}"
       LOG_LEVEL: "${LOG_LEVEL:-INFO}"
-      TRUST_PROXY_HEADERS: "true"
-      SECURE_COOKIES: "true"
+      # Proxies in front of the app (Traefik = 1). The client address and
+      # HTTPS are read from that far in from the right of X-Forwarded-*.
+      TRUSTED_PROXY_HOPS: "1"
     volumes:
       - app_data:/data
     networks: