Honour forwarded headers only from MEALQUEST_TRUSTED_PROXY

Add internal/forwarded: parse the trusted proxy list (comma-separated IPs
and CIDRs, validated at load), resolve each request's client IP and scheme,
and expose them through context helpers. X-Forwarded-For and
X-Forwarded-Proto are honoured only when the TCP peer is trusted; the client
is the rightmost X-Forwarded-For address that is not a trusted proxy.
Malformed forwarded headers from a trusted peer get 400.

The router resolves every request first; a slog handler adds client_ip to
request-scoped log lines. The auth routes refuse requests whose resolved
scheme differs from MEALQUEST_PUBLIC_URL's, so a proxy that is not trusted
fails loudly instead of breaking Secure cookies. The OIDC redirect URI is
still built only from MEALQUEST_PUBLIC_URL.

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

docs/ARCHITECTURE.md

@@ -29,6 +29,15 @@ docker compose project "mealquest" — 10.60.1.14
 One Go service serves the API, the auth flow, media and the PWA. Its SQLite
 database is replicated by Litestream to the NAS.
 
+Traefik terminates TLS. The service honours `X-Forwarded-For` and
+`X-Forwarded-Proto` only from peers listed in `MEALQUEST_TRUSTED_PROXY`
+(comma-separated IPs/CIDRs; production lists `10.60.1.33`); from any other
+peer they are ignored. The client is the rightmost `X-Forwarded-For` address
+that is not a trusted proxy. The resolved client IP is added as `client_ip`
+to every log line written while handling a request, and the auth routes
+refuse requests whose resolved scheme differs from `MEALQUEST_PUBLIC_URL`'s. The OIDC redirect URI is always built
+from `MEALQUEST_PUBLIC_URL`, never from request headers.
+
 ## Repository layout
 
 | Path | Purpose |