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/migrate/migrate.go

@@ -0,0 +1,57 @@
+// Package migrate runs database schema migrations embedded in the binary.
+package migrate
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/golang-migrate/migrate/v4"
+	_ "github.com/golang-migrate/migrate/v4/database/postgres"
+	"github.com/golang-migrate/migrate/v4/source/iofs"
+
+	"osdb/migrations"
+)
+
+func newMigrator(databaseURL string) (*migrate.Migrate, error) {
+	src, err := iofs.New(migrations.FS, ".")
+	if err != nil {
+		return nil, fmt.Errorf("create iofs source: %w", err)
+	}
+
+	m, err := migrate.NewWithSourceInstance("iofs", src, databaseURL)
+	if err != nil {
+		return nil, fmt.Errorf("create migrator: %w", err)
+	}
+
+	return m, nil
+}
+
+// Up applies all pending migrations.
+func Up(databaseURL string) error {
+	m, err := newMigrator(databaseURL)
+	if err != nil {
+		return err
+	}
+	defer m.Close()
+
+	if err := m.Up(); err != nil && !errors.Is(err, migrate.ErrNoChange) {
+		return fmt.Errorf("migrate up: %w", err)
+	}
+
+	return nil
+}
+
+// Down rolls back a single migration step.
+func Down(databaseURL string) error {
+	m, err := newMigrator(databaseURL)
+	if err != nil {
+		return err
+	}
+	defer m.Close()
+
+	if err := m.Steps(-1); err != nil && !errors.Is(err, migrate.ErrNoChange) {
+		return fmt.Errorf("migrate down: %w", err)
+	}
+
+	return nil
+}