refactor(api): consolidate health check endpoints to /health

Removes the redundant `/healthz` and `/api/v1/health` endpoints, leaving `/health` as the sole health check endpoint.

- Update router to return 404 Not Found for the removed endpoints
- Update admin log filtering to only ignore `/health`
- Remove health URL from API documentation data
- Update tests to verify `/health` returns 200 and others return 404
- Update README documentation to reflect the change

backend/libs/handlers/health.go

@@ -13,6 +13,10 @@ type healthResponse struct {
 }
 
 func (a *App) Health(w http.ResponseWriter, r *http.Request) {
+	if r.URL.Path != "/health" {
+		http.NotFound(w, r)
+		return
+	}
 	helpers.WriteJSON(w, http.StatusOK, healthResponse{
 		Status: "ok",
 		Time:   time.Now().UTC().Format(time.RFC3339),