Log fx through slog, snake_case health check keys (closes #27)

fx wrote its own steps of starting and stopping as plain text to
stderr. It now logs them with its slog event logger through the
backend's logger, so off a terminal every line the backend's own
logger and fx write is JSON. A malformed config file now makes
config.New return the error instead of panicking. A test runs the
server as a child process and checks that all it writes is JSON, on
a normal start and stop and with such a config file. The backend
logs its name, version and architecture once at start.

The health check's uptime keys are now uptime_seconds and
uptime_human; its type and method take the names
GO_HTTP_SERVER_CONVENTIONS.md gives.

Model: opus-5-5

backend/cmd/netwatch-server/main.go

@@ -16,6 +16,7 @@ import (
 	"sneak.berlin/go/netwatch/internal/server"
 
 	"go.uber.org/fx"
+	"go.uber.org/fx/fxevent"
 )
 
 //nolint:gochecknoglobals // set via ldflags at build time
@@ -62,6 +63,12 @@ func main() {
 	globals.Version = Version
 
 	fx.New(
+		// fx logs each step of starting and stopping through the
+		// server's own logger, so off a terminal those lines are
+		// JSON like every other line.
+		fx.WithLogger(func(log *logger.Logger) fxevent.Logger {
+			return &fxevent.SlogLogger{Logger: log.Get()}
+		}),
 		fx.Provide(
 			config.New,
 			globals.New,
@@ -72,6 +79,11 @@ func main() {
 			reportbuf.New,
 			server.New,
 		),
-		fx.Invoke(func(*server.Server) {}),
+		fx.Invoke(
+			// First, so the name and version are logged even when
+			// a setting stops the start.
+			func(log *logger.Logger) { log.Identify() },
+			func(*server.Server) {},
+		),
 	).Run()
 }