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/middleware/recovery.go

@@ -0,0 +1,23 @@
+package middleware
+
+import (
+	"log/slog"
+	"net/http"
+	"runtime/debug"
+)
+
+func Recovery(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		defer func() {
+			if err := recover(); err != nil {
+				slog.Error("panic recovered",
+					"error", err,
+					"stack", string(debug.Stack()),
+					"request_id", RequestIDFromCtx(r.Context()),
+				)
+				http.Error(w, `{"error":"internal server error"}`, http.StatusInternalServerError)
+			}
+		}()
+		next.ServeHTTP(w, r)
+	})
+}