Deprecate legacy Login/Register, add admin events, members, webhooks

- Mark Register() and Login() as Deprecated (use Better Auth directly)
- Add admin_events.go, members.go, webhooks.go to match backend API
- Sync types with backend changes

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

auth.go

@@ -28,6 +28,8 @@ func WithOrgName(name string) RegisterOption {
 }
 
 // Register creates a new user and organization.
+//
+// Deprecated: Use Better Auth endpoints directly. See docs.
 func Register(baseURL, email, password string, opts ...RegisterOption) (*AuthRegisterResponse, error) {
 	if baseURL == "" {
 		baseURL = defaultBaseURL
@@ -50,6 +52,8 @@ func Register(baseURL, email, password string, opts ...RegisterOption) (*AuthReg
 }
 
 // Login authenticates a user and returns a session token.
+//
+// Deprecated: Use Better Auth endpoints directly. See docs.
 func Login(baseURL, email, password string) (*AuthLoginResponse, error) {
 	if baseURL == "" {
 		baseURL = defaultBaseURL