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/users/change_password.md

@@ -0,0 +1,39 @@
+# Change Password
+
+Change the current user's password.
+
+**Endpoint**: `PUT /api/users/me/password`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| current_password | string | Yes | Current password |
+| new_password | string | Yes | New password (min 8 chars) |
+
+### Example Request
+
+```json
+{
+  "current_password": "oldPassword",
+  "new_password": "NewSecureP@ss123!"
+}
+```
+
+## Response (204 No Content)
+
+Password changed successfully.
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid input or weak password |
+| 401 | Current password is incorrect |
+| 401 | Invalid or expired token |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->