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>

frontend/src/views/HelloWorld.vue

@@ -0,0 +1,32 @@
+<script setup lang="ts">
+import { ref, onMounted } from 'vue'
+import { getHealth } from '../api/health'
+
+const backendStatus = ref<string | null>(null)
+const backendError = ref<string | null>(null)
+
+onMounted(async () => {
+  try {
+    const data = await getHealth()
+    backendStatus.value = data.status
+  } catch (err) {
+    backendError.value = err instanceof Error ? err.message : 'unknown error'
+  }
+})
+</script>
+
+<template>
+  <main class="flex flex-col items-center justify-center min-h-screen gap-4">
+    <h1 class="text-4xl font-bold">Hello, OSDB!</h1>
+
+    <div v-if="backendStatus" class="text-green-600">
+      Backend: {{ backendStatus }}
+    </div>
+    <div v-else-if="backendError" class="text-red-600">
+      Backend unreachable: {{ backendError }}
+    </div>
+    <div v-else class="text-gray-400">
+      Checking backend…
+    </div>
+  </main>
+</template>