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/migrations/000001_init.down.sql

@@ -0,0 +1,2 @@
+-- 000001_init.down.sql: rollback for baseline migration (no-op)
+SELECT 1;

backend/migrations/000001_init.up.sql

@@ -0,0 +1,3 @@
+-- 000001_init.up.sql: baseline migration (no-op)
+-- Real schema starts in story #02.
+SELECT 1;

backend/migrations/embed.go

@@ -0,0 +1,9 @@
+// Package migrations embeds SQL migration files for use with golang-migrate.
+package migrations
+
+import "embed"
+
+// FS holds all *.sql migration files embedded at compile time.
+//
+//go:embed *.sql
+var FS embed.FS