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>

internal/server/origin.go

@@ -0,0 +1,34 @@
+package server
+
+import (
+	"errors"
+	"log/slog"
+	"net/http"
+
+	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/forwarded"
+)
+
+// resolveOrigin resolves every request's client address and scheme with
+// forwarded.Resolve and puts them in the context (read them with
+// forwarded.ClientIP and forwarded.Scheme). Forwarded headers are honoured
+// only from a peer in trusted. A malformed forwarded header from a trusted
+// peer gets 400 bad_request.
+func resolveOrigin(logger *slog.Logger, trusted forwarded.Trusted) func(http.Handler) http.Handler {
+	badRequest := apiErrorHandler(logger, http.StatusBadRequest, "bad_request", "invalid forwarded headers")
+	internal := apiErrorHandler(logger, http.StatusInternalServerError, "internal", "internal error")
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			o, err := forwarded.Resolve(r, trusted)
+			switch {
+			case errors.Is(err, forwarded.ErrInvalidHeader):
+				logger.WarnContext(r.Context(), "rejected forwarded headers", "peer", r.RemoteAddr, "error", err)
+				badRequest(w, r)
+			case err != nil:
+				logger.ErrorContext(r.Context(), "resolve request origin", "error", err)
+				internal(w, r)
+			default:
+				next.ServeHTTP(w, r.WithContext(forwarded.NewContext(r.Context(), o)))
+			}
+		})
+	}
+}