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>

README.md

@@ -28,10 +28,10 @@ services:
     environment:
       DATABASE_URL: "postgresql+asyncpg://${POSTGRES_USER:-grimoire}:${POSTGRES_PASSWORD:-grimoire}@db:5432/${POSTGRES_DB:-grimoire}"
       LOG_LEVEL: "${LOG_LEVEL:-INFO}"
-      # Behind an HTTPS reverse proxy:
+      # Port 5000 is published straight to clients, so no proxy is trusted.
+      # Behind a reverse proxy, set 1 (one per proxy) and BASE_URL:
+      TRUSTED_PROXY_HOPS: "0"
       # BASE_URL: "https://grimoire.example.com"
-      # SECURE_COOKIES: "true"
-      # TRUST_PROXY_HEADERS: "true"
     volumes:
       - app_data:/data
     depends_on:
@@ -75,8 +75,7 @@ Everything is optional. Set these in the environment or in a `.env` file beside
 | Variable | When | What it does |
 |----------|------|--------------|
 | `BASE_URL` | Behind a proxy | The public URL, used in email links and SSO redirects |
-| `SECURE_COOKIES` | Over HTTPS | `true` when Grimoire is served over HTTPS |
-| `TRUST_PROXY_HEADERS` | Behind a proxy | `true` to read the client address from the proxy's headers |
+| `TRUSTED_PROXY_HOPS` | Behind a proxy | How many reverse proxies stand in front of Grimoire (default `1`; the compose file above sets `0` because it publishes the port directly). The client address and HTTPS are read from the proxy's headers by it |
 | `POSTGRES_PASSWORD` | Before the first start | The database password (defaults to `grimoire`; the database is reachable only inside the compose network). Postgres takes it once, when the volume is created |
 | `SECRET_KEY` | Rarely | A key you manage yourself, instead of the one Grimoire keeps. It wins over the kept key; remove it to go back |
 
@@ -132,7 +131,7 @@ The full list is in [Features](docs/features.md).
 
 | What you see | Why | Fix |
 |--------------|-----|-----|
-| `SECRET_KEY is set to the public placeholder` at startup | `SECURE_COOKIES=true` while `SECRET_KEY` is `dev-secret-change-me` | Remove `SECRET_KEY`, or set a real one |
+| `SECRET_KEY is set to the public placeholder` at startup | `SECRET_KEY` is `dev-secret-change-me` or `change-me-in-production` | Remove `SECRET_KEY`, or set a real one |
 | Settings asks for forge tokens or the backup secret again | The instance's key changed: the `app_data` volume was replaced, or `SECRET_KEY` was set or removed | Enter them again, or restore the previous key |
 | `Can't locate revision identified by '…'` at startup | An older image is running against a database a newer one has already migrated | Run the newer image; migrations only move forward |
 | The plugin's tools are missing in Claude Code | The plugin isn't installed, or Claude Code hasn't reloaded it | `/plugin` to install or update it, then `/reload-plugins` |