Honour forwarded headers only from trusted proxies (closes #259)

Before, the server took the client's address from X-Forwarded-For or X-Real-IP whoever sent them, so a direct client could get past the login rate limit and choose the address and hostname recorded for its session.

A new ClientIP middleware, run before request logging, resolves the address once: forwarded headers count only when the peer is in TRUSTED_PROXIES, which defaults to the RFC 1918 ranges. A set list replaces the default, the empty string trusts nothing, and an invalid entry stops startup. The request log, login rate limiter and session creation all use the resolved address.

A proxy on the same host as a server outside Docker connects from 127.0.0.1 or ::1, which the default does not trust; the README says to list it.

Model: opus-5-5

internal/handlers/api.go

@@ -42,25 +42,9 @@ func (hdlr *Handlers) maxBodySize() int64 {
 	return defaultMaxBodySize
 }
 
-// clientIP extracts the connecting client's IP address
-// from the request, checking X-Forwarded-For and
-// X-Real-IP headers before falling back to RemoteAddr.
+// clientIP returns the IP address in request.RemoteAddr, which the
+// ClientIP middleware has set to the client's address.
 func clientIP(request *http.Request) string {
-	if forwarded := request.Header.Get("X-Forwarded-For"); forwarded != "" {
-		// X-Forwarded-For can contain a comma-separated list;
-		// the first entry is the original client.
-		parts := strings.SplitN(forwarded, ",", 2) //nolint:mnd
-		ip := strings.TrimSpace(parts[0])
-
-		if ip != "" {
-			return ip
-		}
-	}
-
-	if realIP := request.Header.Get("X-Real-IP"); realIP != "" {
-		return strings.TrimSpace(realIP)
-	}
-
 	host, _, err := net.SplitHostPort(request.RemoteAddr)
 	if err != nil {
 		return request.RemoteAddr