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.
Rules suppressed: revive and tagliatelle on HealthcheckResponse, whose name and keys come from the conventions; gosec where the test starts its own binary.

Model: opus-5-5

backend/internal/config/config.go

@@ -73,7 +73,8 @@ type Config struct {
 
 // New loads configuration from env, .env files, and config
 // files, returning a fully resolved Config. It fails, with an error
-// naming the setting, on a value the server cannot use.
+// naming the setting, on a value the server cannot use, and on a
+// config file it finds but cannot read.
 func New(
 	_ fx.Lifecycle,
 	params Params,
@@ -106,8 +107,8 @@ func New(
 	if err != nil {
 		var notFound viper.ConfigFileNotFoundError
 		if !errors.As(err, &notFound) {
-			log.Error("config file malformed", "error", err)
-			panic(err)
+			return nil, fmt.Errorf("config file %s: %w",
+				viper.ConfigFileUsed(), err)
 		}
 	}