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>

Makefile

@@ -0,0 +1,40 @@
+include .env
+export
+
+.PHONY: dev-db migrate-up migrate-down run-backend run-frontend test fmt
+
+## dev-db: start postgres and block until healthy
+dev-db:
+	docker compose up -d db
+	@echo "Waiting for Postgres to be healthy..."
+	@until [ "$$(docker inspect --format='{{.State.Health.Status}}' osdb-db 2>/dev/null)" = "healthy" ]; do \
+		echo "  still starting..."; \
+		sleep 2; \
+	done
+	@echo "Postgres is healthy."
+
+## migrate-up: apply all pending migrations
+migrate-up:
+	cd backend && go run ./cmd/server migrate
+
+## migrate-down: roll back one migration step
+migrate-down:
+	cd backend && go run ./cmd/server migrate down
+
+## run-backend: start the Echo API server
+run-backend:
+	cd backend && go run ./cmd/server
+
+## run-frontend: start the Vite dev server
+run-frontend:
+	cd frontend && npm run dev
+
+## test: run all backend and frontend tests
+test:
+	cd backend && go test ./...
+	cd frontend && npm run test -- --run
+
+## fmt: format all code
+fmt:
+	cd backend && gofmt -w .
+	cd frontend && npx prettier --write .