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, and logs fx's "received signal"
line as Run did. 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: