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/user/service.go

@@ -0,0 +1,117 @@
+package user
+
+import (
+	"context"
+	"fmt"
+)
+
+type Service struct {
+	repo UserRepository
+}
+
+func NewService(repo UserRepository) *Service {
+	return &Service{repo: repo}
+}
+
+func (s *Service) GetProfile(ctx context.Context, userID string) (*User, error) {
+	return s.repo.GetByID(ctx, userID)
+}
+
+func (s *Service) UpdateProfile(ctx context.Context, userID string, req UpdateProfileRequest) (*User, error) {
+	if err := validateProfileRequest(req); err != nil {
+		return nil, err
+	}
+
+	if !req.HasBodyParams() {
+		updated, err := s.repo.Update(ctx, userID, req)
+		if err != nil {
+			return nil, fmt.Errorf("update profile: %w", err)
+		}
+		return updated, nil
+	}
+
+	// Need to update profile + recalculate calories in a single transaction.
+	// First, get current user to merge with incoming fields for calorie calculation.
+	current, err := s.repo.GetByID(ctx, userID)
+	if err != nil {
+		return nil, fmt.Errorf("get user: %w", err)
+	}
+
+	// Merge current values with request to compute calories
+	height := current.HeightCM
+	if req.HeightCM != nil {
+		height = req.HeightCM
+	}
+	weight := current.WeightKG
+	if req.WeightKG != nil {
+		weight = req.WeightKG
+	}
+	age := current.Age
+	if req.Age != nil {
+		age = req.Age
+	}
+	gender := current.Gender
+	if req.Gender != nil {
+		gender = req.Gender
+	}
+	activity := current.Activity
+	if req.Activity != nil {
+		activity = req.Activity
+	}
+	goal := current.Goal
+	if req.Goal != nil {
+		goal = req.Goal
+	}
+
+	calories := CalculateDailyCalories(height, weight, age, gender, activity, goal)
+
+	var calReq *UpdateProfileRequest
+	if calories != nil {
+		calReq = &UpdateProfileRequest{DailyCalories: calories}
+	}
+
+	updated, err := s.repo.UpdateInTx(ctx, userID, req, calReq)
+	if err != nil {
+		return nil, fmt.Errorf("update profile: %w", err)
+	}
+
+	return updated, nil
+}
+
+func validateProfileRequest(req UpdateProfileRequest) error {
+	if req.HeightCM != nil {
+		if *req.HeightCM < 100 || *req.HeightCM > 250 {
+			return fmt.Errorf("height_cm must be between 100 and 250")
+		}
+	}
+	if req.WeightKG != nil {
+		if *req.WeightKG < 30 || *req.WeightKG > 300 {
+			return fmt.Errorf("weight_kg must be between 30 and 300")
+		}
+	}
+	if req.Age != nil {
+		if *req.Age < 10 || *req.Age > 120 {
+			return fmt.Errorf("age must be between 10 and 120")
+		}
+	}
+	if req.Gender != nil {
+		if *req.Gender != "male" && *req.Gender != "female" {
+			return fmt.Errorf("gender must be 'male' or 'female'")
+		}
+	}
+	if req.Activity != nil {
+		switch *req.Activity {
+		case "low", "moderate", "high":
+		default:
+			return fmt.Errorf("activity must be 'low', 'moderate', or 'high'")
+		}
+	}
+	if req.Goal != nil {
+		switch *req.Goal {
+		case "lose", "maintain", "gain":
+		default:
+			return fmt.Errorf("goal must be 'lose', 'maintain', or 'gain'")
+		}
+	}
+	return nil
+}