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/login.md

@@ -0,0 +1,49 @@
+# Login User
+
+Authenticate with email and password.
+
+**Endpoint**: `POST /api/auth/login`
+**Auth**: Not required
+**Content-Type**: `application/json`
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| email | string | Yes | User's email address |
+| password | string | Yes | User's password |
+
+### Example Request
+
+```json
+{
+  "email": "user@example.com",
+  "password": "SecureP@ss123!"
+}
+```
+
+## Response (200 OK)
+
+```json
+{
+  "token": "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...",
+  "refresh_token": "d4f5g6h7...",
+  "user": {
+    "id": "uuid-here",
+    "email": "user@example.com",
+    "username": "john",
+    "role": "user"
+  }
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 401 | Invalid email or password |
+| 400 | Missing required fields |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->