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 own slog event logger through the
backend's logger, so off a terminal every line from start to stop is
JSON. The backend logs its name, version and architecture once at
start, through the logger's Identify, which nothing called before.

The health check's uptime keys are now uptime_seconds and
uptime_human, and its type and method take the names
GO_HTTP_SERVER_CONVENTIONS.md gives, HealthcheckResponse and
Healthcheck(). Its path, content type, status and code are unchanged.

Model: opus-5-5

TODO.md

@@ -23,6 +23,14 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-10-03: the backend's logs are one stream (issue #27): fx logs its own
+  steps of starting and stopping through the backend's logger, so off a terminal
+  every line from start to stop is JSON, where fx used to write plain text to
+  stderr. 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
+  path, content type, `"status":"ok"` and 200 are unchanged. `SENTRY_DSN`,
+  `METRICS_USERNAME` and `METRICS_PASSWORD` are still read and still unused, and
+  left out of `backend/README.md`, until issues #94 and #95 wire them up
 - 2026-10-03: root no longer acts outside `/data` when it prepares `DATA_DIR`
   (issue #80): `bin/entrypoint.sh` runs `netwatch-server prepare-data-dir`,
   which refuses a `DATA_DIR` that is not `/data` or a path below it written in

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()
 }

backend/internal/handlers/healthcheck.go

@@ -6,6 +6,6 @@ import "net/http"
 // endpoint.
 func (s *Handlers) HandleHealthCheck() http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
-		s.respondJSON(w, r, s.hc.Check(), http.StatusOK)
+		s.respondJSON(w, r, s.hc.Healthcheck(), http.StatusOK)
 	}
 }

backend/internal/handlers/healthcheck_test.go

@@ -50,8 +50,8 @@ func newStartedHandlers(t *testing.T, g *globals.Globals) *handlers.Handlers {
 
 // TestHandleHealthCheck checks the health check's answer: 200, a JSON
 // content type, and a JSON object with exactly the fields of
-// healthcheck.Response, carrying this server's name and version and
-// an uptime counted from its start.
+// healthcheck.HealthcheckResponse, carrying this server's name and
+// version and an uptime counted from its start.
 func TestHandleHealthCheck(t *testing.T) {
 	t.Parallel()
 
@@ -82,7 +82,7 @@ func TestHandleHealthCheck(t *testing.T) {
 	}
 
 	fields := []string{
-		"appname", "now", "status", "uptimeHuman", "uptimeSeconds", "version",
+		"appname", "now", "status", "uptime_human", "uptime_seconds", "version",
 	}
 	if got := slices.Sorted(maps.Keys(body)); !slices.Equal(got, fields) {
 		t.Fatalf("fields = %v, want %v", got, fields)
@@ -104,17 +104,17 @@ func TestHandleHealthCheck(t *testing.T) {
 	}
 
 	// Started just now, so the uptime is well under a minute.
-	human, _ := body["uptimeHuman"].(string)
+	human, _ := body["uptime_human"].(string)
 
 	uptime, err := time.ParseDuration(human)
 	if err != nil || uptime > time.Minute {
-		t.Errorf("uptimeHuman = %q, want a duration under a minute (%v)",
+		t.Errorf("uptime_human = %q, want a duration under a minute (%v)",
 			human, err)
 	}
 
-	seconds, ok := body["uptimeSeconds"].(float64)
+	seconds, ok := body["uptime_seconds"].(float64)
 	if !ok || seconds < 0 || seconds > time.Minute.Seconds() {
-		t.Errorf("uptimeSeconds = %v, want a number of seconds under a minute",
-			body["uptimeSeconds"])
+		t.Errorf("uptime_seconds = %v, want a number of seconds under a minute",
+			body["uptime_seconds"])
 	}
 }

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",