docs: scaffold API.md with section headings

docs/API.md

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+# Xpensely Server — API Reference
+
+> Last updated: 2026-05-09 · Branch: `feature/security-hardening`
+
+## Table of Contents
+1. [Overview](#1-overview)
+2. [Authentication](#2-authentication)
+3. [Rate Limiting](#3-rate-limiting)
+4. [Endpoints](#4-endpoints)
+   - 4.1 [Home](#41-home)
+   - 4.2 [Users](#42-users)
+   - 4.3 [Expense Lists](#43-expense-lists)
+5. [Data Models](#5-data-models)
+6. [Error Handling](#6-error-handling)
+7. [Recent Changes — `feature/security-hardening`](#7-recent-changes)
+
+---
+
+## 1. Overview
+
+_TODO_
+
+## 2. Authentication
+
+_TODO_
+
+## 3. Rate Limiting
+
+_TODO_
+
+## 4. Endpoints
+
+### 4.1 Home
+
+_TODO_
+
+### 4.2 Users
+
+_TODO_
+
+### 4.3 Expense Lists
+
+_TODO_
+
+## 5. Data Models
+
+_TODO_
+
+## 6. Error Handling
+
+_TODO_
+
+## 7. Recent Changes — `feature/security-hardening`
+
+_TODO_