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/auth/jwt.go

@@ -0,0 +1,69 @@
+package auth
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/golang-jwt/jwt/v5"
+	"github.com/google/uuid"
+)
+
+type JWTManager struct {
+	secret          []byte
+	accessDuration  time.Duration
+	refreshDuration time.Duration
+}
+
+type Claims struct {
+	UserID string `json:"user_id"`
+	Plan   string `json:"plan"`
+	jwt.RegisteredClaims
+}
+
+func NewJWTManager(secret string, accessDuration, refreshDuration time.Duration) *JWTManager {
+	return &JWTManager{
+		secret:          []byte(secret),
+		accessDuration:  accessDuration,
+		refreshDuration: refreshDuration,
+	}
+}
+
+func (j *JWTManager) GenerateAccessToken(userID, plan string) (string, error) {
+	claims := Claims{
+		UserID: userID,
+		Plan:   plan,
+		RegisteredClaims: jwt.RegisteredClaims{
+			ExpiresAt: jwt.NewNumericDate(time.Now().Add(j.accessDuration)),
+			IssuedAt:  jwt.NewNumericDate(time.Now()),
+		},
+	}
+	token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
+	return token.SignedString(j.secret)
+}
+
+func (j *JWTManager) GenerateRefreshToken() (string, time.Time) {
+	token := uuid.NewString()
+	expiresAt := time.Now().Add(j.refreshDuration)
+	return token, expiresAt
+}
+
+func (j *JWTManager) ValidateAccessToken(tokenStr string) (*Claims, error) {
+	token, err := jwt.ParseWithClaims(tokenStr, &Claims{}, func(t *jwt.Token) (interface{}, error) {
+		if _, ok := t.Method.(*jwt.SigningMethodHMAC); !ok {
+			return nil, fmt.Errorf("unexpected signing method: %v", t.Header["alg"])
+		}
+		return j.secret, nil
+	})
+	if err != nil {
+		return nil, err
+	}
+	claims, ok := token.Claims.(*Claims)
+	if !ok || !token.Valid {
+		return nil, fmt.Errorf("invalid token")
+	}
+	return claims, nil
+}
+
+func (j *JWTManager) AccessDuration() time.Duration {
+	return j.accessDuration
+}