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/healthcheck/healthcheck.go

@@ -30,14 +30,17 @@ type Healthcheck struct {
 	params      *Params
 }
 
-// Response is the JSON payload returned by the health check
-// endpoint.
-type Response struct {
+// HealthcheckResponse is the JSON payload returned by the health
+// check endpoint. Its name and its snake_case keys are the ones
+// GO_HTTP_SERVER_CONVENTIONS.md gives.
+//
+//nolint:revive,tagliatelle // name and keys from the conventions
+type HealthcheckResponse struct {
 	Appname       string `json:"appname"`
 	Now           string `json:"now"`
 	Status        string `json:"status"`
-	UptimeHuman   string `json:"uptimeHuman"`
-	UptimeSeconds int64  `json:"uptimeSeconds"`
+	UptimeHuman   string `json:"uptime_human"`
+	UptimeSeconds int64  `json:"uptime_seconds"`
 	Version       string `json:"version"`
 }
 
@@ -65,9 +68,9 @@ func New(
 	return s, nil
 }
 
-// Check returns the current health status of the application.
-func (s *Healthcheck) Check() *Response {
-	return &Response{
+// Healthcheck returns the current health status of the application.
+func (s *Healthcheck) Healthcheck() *HealthcheckResponse {
+	return &HealthcheckResponse{
 		Appname:       s.params.Globals.Appname,
 		Now:           time.Now().UTC().Format(time.RFC3339Nano),
 		Status:        "ok",