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.go

@@ -0,0 +1,20 @@
+package handler
+
+import (
+	"net/http"
+
+	"github.com/labstack/echo/v4"
+)
+
+// HealthHandler serves the health check endpoint.
+type HealthHandler struct{}
+
+// NewHealthHandler returns a new HealthHandler.
+func NewHealthHandler() *HealthHandler {
+	return &HealthHandler{}
+}
+
+// Health responds with {"status":"ok"}.
+func (h *HealthHandler) Health(c echo.Context) error {
+	return c.JSON(http.StatusOK, map[string]string{"status": "ok"})
+}