feat: implement Iteration 0 foundation (backend + Flutter client)

Backend (Go):
- Project structure with chi router, pgxpool, goose migrations
- JWT auth (access/refresh tokens) with Firebase token verification
- NoopTokenVerifier for local dev without Firebase credentials
- PostgreSQL user repository with atomic profile updates (transactions)
- Mifflin-St Jeor calorie calculation based on profile data
- REST API: POST /auth/login, /auth/refresh, /auth/logout, GET/PUT /profile, GET /health
- Middleware: auth, CORS (localhost wildcard), logging, recovery, request_id
- Unit tests (51 passing) and integration tests (testcontainers)
- Docker Compose setup with postgres healthcheck and graceful shutdown

Flutter client:
- Riverpod state management with GoRouter navigation
- Firebase Auth (email/password + Google sign-in with web popup support)
- Platform-aware API URLs (web/Android/iOS)
- Dio HTTP client with JWT auth interceptor and concurrent refresh handling
- Secure token storage
- Screens: Login, Register, Home (tabs: Menu, Recipes, Products, Profile)
- Unit tests (17 passing)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/testutil/postgres.go

@@ -0,0 +1,77 @@
+package testutil
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+	"testing"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+	_ "github.com/jackc/pgx/v5/stdlib"
+	"github.com/pressly/goose/v3"
+	"github.com/testcontainers/testcontainers-go"
+	"github.com/testcontainers/testcontainers-go/wait"
+)
+
+func SetupTestDB(t *testing.T) *pgxpool.Pool {
+	t.Helper()
+	ctx := context.Background()
+
+	container, err := testcontainers.GenericContainer(ctx, testcontainers.GenericContainerRequest{
+		ContainerRequest: testcontainers.ContainerRequest{
+			Image:        "postgres:16-alpine",
+			ExposedPorts: []string{"5432/tcp"},
+			Env: map[string]string{
+				"POSTGRES_DB":       "test_db",
+				"POSTGRES_USER":     "test_user",
+				"POSTGRES_PASSWORD": "test_pass",
+			},
+			WaitingFor: wait.ForListeningPort("5432/tcp"),
+		},
+		Started: true,
+	})
+	if err != nil {
+		t.Fatalf("failed to start container: %v", err)
+	}
+
+	t.Cleanup(func() {
+		container.Terminate(ctx)
+	})
+
+	host, err := container.Host(ctx)
+	if err != nil {
+		t.Fatalf("failed to get host: %v", err)
+	}
+
+	port, err := container.MappedPort(ctx, "5432")
+	if err != nil {
+		t.Fatalf("failed to get port: %v", err)
+	}
+
+	dsn := fmt.Sprintf("postgres://test_user:test_pass@%s:%s/test_db?sslmode=disable", host, port.Port())
+
+	// Run migrations with goose
+	db, err := sql.Open("pgx", dsn)
+	if err != nil {
+		t.Fatalf("failed to open db for migrations: %v", err)
+	}
+	defer db.Close()
+
+	if err := goose.Up(db, "../../../migrations"); err != nil {
+		// Try relative path from different test locations
+		if err2 := goose.Up(db, "../../migrations"); err2 != nil {
+			t.Fatalf("failed to run migrations: %v (also tried: %v)", err, err2)
+		}
+	}
+
+	pool, err := pgxpool.New(ctx, dsn)
+	if err != nil {
+		t.Fatalf("failed to create pool: %v", err)
+	}
+
+	t.Cleanup(func() {
+		pool.Close()
+	})
+
+	return pool
+}