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

go.mod

@@ -16,6 +16,7 @@ require (
 	github.com/prometheus/client_golang v1.24.1
 	github.com/slok/go-http-metrics v0.13.0
 	github.com/spf13/viper v1.21.0
+	go.uber.org/dig v1.19.0
 	go.uber.org/fx v1.24.0
 	golang.org/x/sys v0.47.0
 )
@@ -38,7 +39,6 @@ require (
 	github.com/spf13/pflag v1.0.10 // indirect
 	github.com/subosito/gotenv v1.6.0 // indirect
 	github.com/zeebo/xxh3 v1.0.2 // indirect
-	go.uber.org/dig v1.19.0 // indirect
 	go.uber.org/multierr v1.10.0 // indirect
 	go.uber.org/zap v1.26.0 // indirect
 	go.yaml.in/yaml/v3 v3.0.4 // indirect