Log fx's startup steps at debug; a refused setting is one line (closes #209)

fx's event lines, errors included, now log at debug level, so a normal
start shows none and DEBUG shows them. RunServer starts and stops the
fx app itself instead of calling Run: when the start fails, it logs the
error's root cause as one "cannot start" line, the backend's last, and
returns 1. Main returns the server's exit code.

Model: opus-5-5

README.md

@@ -336,6 +336,10 @@ The backend logs JSON to stdout, or plain text when stdout is a terminal. Each
 request is logged with its method, URL, status, client address and duration.
 Every value taken from a request is cut to 128 bytes before it is logged.
 
+The steps `fx` takes to start and stop the backend are logged at debug level, so
+they show only with `DEBUG` set. When the backend refuses a setting, its last
+log line is one error that names the setting and the reason.
+
 ## Design
 
 The repository holds the page, the backend and the image: