feat: bootstrap OSDB full-stack skeleton (story #01)

- Go backend: Echo v4 + pgxpool + embedded golang-migrate; health endpoints
  at /health and /api/v1/health; TDD health handler (httptest)
- Vue 3 frontend: Vite + TypeScript + Pinia + vue-router + Tailwind CSS v4;
  TDD HelloWorld component (@vue/test-utils + jsdom + vitest)
- Infra: docker-compose postgres:16 with env-interpolated credentials;
  Makefile with dev-db health-wait loop, migrate-up/down, run, test, fmt
- embed.FS migrations at backend/migrations/ (000001 no-op baseline)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/handler/health_handler_test.go

@@ -0,0 +1,36 @@
+package handler_test
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/labstack/echo/v4"
+	"osdb/internal/handler"
+)
+
+func TestHealth(t *testing.T) {
+	e := echo.New()
+	req := httptest.NewRequest(http.MethodGet, "/health", nil)
+	rec := httptest.NewRecorder()
+	c := e.NewContext(req, rec)
+
+	h := handler.NewHealthHandler()
+	if err := h.Health(c); err != nil {
+		t.Fatalf("Health() returned error: %v", err)
+	}
+
+	if rec.Code != http.StatusOK {
+		t.Errorf("expected status 200, got %d", rec.Code)
+	}
+
+	var body map[string]string
+	if err := json.Unmarshal(rec.Body.Bytes(), &body); err != nil {
+		t.Fatalf("failed to parse response body: %v", err)
+	}
+
+	if got := body["status"]; got != "ok" {
+		t.Errorf("expected status=ok, got %q", got)
+	}
+}