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/model.go

@@ -0,0 +1,44 @@
+package user
+
+import (
+	"encoding/json"
+	"time"
+)
+
+type User struct {
+	ID            string          `json:"id"`
+	FirebaseUID   string          `json:"-"`
+	Email         string          `json:"email"`
+	Name          string          `json:"name"`
+	AvatarURL     *string         `json:"avatar_url"`
+	HeightCM      *int            `json:"height_cm"`
+	WeightKG      *float64        `json:"weight_kg"`
+	Age           *int            `json:"age"`
+	Gender        *string         `json:"gender"`
+	Activity      *string         `json:"activity"`
+	Goal          *string         `json:"goal"`
+	DailyCalories *int            `json:"daily_calories"`
+	Plan          string          `json:"plan"`
+	Preferences   json.RawMessage `json:"preferences"`
+	CreatedAt     time.Time       `json:"created_at"`
+	UpdatedAt     time.Time       `json:"updated_at"`
+}
+
+type UpdateProfileRequest struct {
+	Name          *string          `json:"name"`
+	HeightCM      *int             `json:"height_cm"`
+	WeightKG      *float64         `json:"weight_kg"`
+	Age           *int             `json:"age"`
+	Gender        *string          `json:"gender"`
+	Activity      *string          `json:"activity"`
+	Goal          *string          `json:"goal"`
+	Preferences   *json.RawMessage `json:"preferences"`
+	DailyCalories *int             `json:"-"` // internal, set by service
+}
+
+// HasBodyParams returns true if any body parameter is being updated
+// that would require recalculation of daily calories.
+func (r *UpdateProfileRequest) HasBodyParams() bool {
+	return r.HeightCM != nil || r.WeightKG != nil || r.Age != nil ||
+		r.Gender != nil || r.Activity != nil || r.Goal != nil
+}