docs: add authentication and user management API endpoints

Phase 2 part 2: Split auth and user endpoints
- Authentication: register, login, refresh_token, logout
- Users: get_profile, update_profile, update_theme, change_password
- Each endpoint in separate markdown file
- Include request/response examples and error codes

docs/api/authentication/refresh_token.md

@@ -0,0 +1,41 @@
+# Refresh Token
+
+Obtain a new JWT token using a refresh token.
+
+**Endpoint**: `POST /api/auth/refresh`
+**Auth**: Not required (uses refresh token)
+**Content-Type**: `application/json`
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| refresh_token | string | Yes | Valid refresh token |
+
+### Example Request
+
+```json
+{
+  "refresh_token": "d4f5g6h7..."
+}
+```
+
+## Response (200 OK)
+
+```json
+{
+  "token": "new-jwt-token",
+  "refresh_token": "new-refresh-token"
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 401 | Invalid or expired refresh token |
+| 400 | Missing refresh token |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->