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/handler.go

@@ -0,0 +1,78 @@
+package user
+
+import (
+	"encoding/json"
+	"log/slog"
+	"net/http"
+
+	"github.com/food-ai/backend/internal/middleware"
+)
+
+const maxRequestBodySize = 1 << 20 // 1 MB
+
+type Handler struct {
+	service *Service
+}
+
+func NewHandler(service *Service) *Handler {
+	return &Handler{service: service}
+}
+
+func (h *Handler) Get(w http.ResponseWriter, r *http.Request) {
+	userID := middleware.UserIDFromCtx(r.Context())
+	if userID == "" {
+		writeErrorJSON(w, http.StatusUnauthorized, "unauthorized")
+		return
+	}
+
+	u, err := h.service.GetProfile(r.Context(), userID)
+	if err != nil {
+		writeErrorJSON(w, http.StatusNotFound, "user not found")
+		return
+	}
+
+	writeJSON(w, http.StatusOK, u)
+}
+
+func (h *Handler) Update(w http.ResponseWriter, r *http.Request) {
+	userID := middleware.UserIDFromCtx(r.Context())
+	if userID == "" {
+		writeErrorJSON(w, http.StatusUnauthorized, "unauthorized")
+		return
+	}
+
+	r.Body = http.MaxBytesReader(w, r.Body, maxRequestBodySize)
+	var req UpdateProfileRequest
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		writeErrorJSON(w, http.StatusBadRequest, "invalid request body")
+		return
+	}
+
+	u, err := h.service.UpdateProfile(r.Context(), userID, req)
+	if err != nil {
+		writeErrorJSON(w, http.StatusBadRequest, err.Error())
+		return
+	}
+
+	writeJSON(w, http.StatusOK, u)
+}
+
+type errorResponse struct {
+	Error string `json:"error"`
+}
+
+func writeErrorJSON(w http.ResponseWriter, status int, msg string) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	if err := json.NewEncoder(w).Encode(errorResponse{Error: msg}); err != nil {
+		slog.Error("failed to write error response", "err", err)
+	}
+}
+
+func writeJSON(w http.ResponseWriter, status int, v interface{}) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	if err := json.NewEncoder(w).Encode(v); err != nil {
+		slog.Error("failed to write JSON response", "err", err)
+	}
+}