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/migrations/001_create_users.sql

@@ -0,0 +1,49 @@
+-- +goose Up
+CREATE EXTENSION IF NOT EXISTS "uuid-ossp";
+
+CREATE TYPE user_plan AS ENUM ('free', 'paid');
+CREATE TYPE user_gender AS ENUM ('male', 'female');
+CREATE TYPE user_goal AS ENUM ('lose', 'maintain', 'gain');
+CREATE TYPE activity_level AS ENUM ('low', 'moderate', 'high');
+
+CREATE TABLE users (
+    id            UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
+    firebase_uid  VARCHAR(128) NOT NULL UNIQUE,
+    email         VARCHAR(255) NOT NULL,
+    name          VARCHAR(255) NOT NULL DEFAULT '',
+    avatar_url    TEXT,
+
+    -- Body parameters
+    height_cm     SMALLINT,
+    weight_kg     DECIMAL(5,2),
+    age           SMALLINT,
+    gender        user_gender,
+    activity      activity_level,
+
+    -- Goal and calculated daily norm
+    goal          user_goal,
+    daily_calories INTEGER,
+
+    -- Plan
+    plan          user_plan NOT NULL DEFAULT 'free',
+
+    -- Preferences (JSONB for flexibility)
+    preferences   JSONB NOT NULL DEFAULT '{}'::jsonb,
+
+    -- Refresh token
+    refresh_token TEXT,
+    token_expires_at TIMESTAMPTZ,
+
+    created_at    TIMESTAMPTZ NOT NULL DEFAULT now(),
+    updated_at    TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE INDEX idx_users_firebase_uid ON users (firebase_uid);
+CREATE INDEX idx_users_email ON users (email);
+
+-- +goose Down
+DROP TABLE IF EXISTS users;
+DROP TYPE IF EXISTS activity_level;
+DROP TYPE IF EXISTS user_goal;
+DROP TYPE IF EXISTS user_gender;
+DROP TYPE IF EXISTS user_plan;