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/Makefile

@@ -0,0 +1,45 @@
+.PHONY: run test lint migrate-up migrate-down migrate-create migrate-status docker-up docker-down docker-logs
+
+ifneq (,$(wildcard .env))
+  include .env
+  export
+endif
+
+# Run for development
+run:
+	go run ./cmd/server
+
+# Tests
+test:
+	go test ./... -v -race -count=1
+
+# Integration tests (require Docker)
+test-integration:
+	go test -tags=integration ./... -v -race -count=1
+
+# Linter
+lint:
+	golangci-lint run ./...
+
+# Migrations
+migrate-up:
+	goose -dir migrations postgres "$(DATABASE_URL)" up
+
+migrate-down:
+	goose -dir migrations postgres "$(DATABASE_URL)" down
+
+migrate-create:
+	goose -dir migrations create $(name) sql
+
+migrate-status:
+	goose -dir migrations postgres "$(DATABASE_URL)" status
+
+# Docker
+docker-up:
+	docker compose up -d
+
+docker-down:
+	docker compose down
+
+docker-logs:
+	docker compose logs -f app