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/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"])
 	}
 }