feat: add profile REST endpoints with HR zone validation

Implements GET /api/profile and PUT /api/profile endpoints with validation
of HR zones (must be contiguous, non-overlapping, 0-100%). Also updates
profile migration to use correct HR zone defaults (0-20, 20-40, etc.)
that match validation requirements.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/internal/api/server.go

@@ -40,6 +40,11 @@ func (s *Server) Router() http.Handler {
 	r.Route("/api", func(r chi.Router) {
 		r.Get("/health", s.handleHealth)
 
+		r.Route("/profile", func(r chi.Router) {
+			r.Get("/", s.handleGetProfile)
+			r.Put("/", s.handleUpdateProfile)
+		})
+
 		r.Route("/auth", func(r chi.Router) {
 			r.Post("/login", s.handleAuthLogin)
 			r.Post("/mfa", s.handleAuthMFA)