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

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+# 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 -->

docs/api/authentication/logout.md

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+# Logout User
+
+Invalidate the current JWT token.
+
+**Endpoint**: `POST /api/auth/logout`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Request Headers
+
+| Header | Type | Required | Description |
+|--------|------|-----------|-------------|
+| Authorization | string | Yes | Bearer token (e.g., `Bearer eyJhbG...`) |
+
+### Example Request
+
+```http
+POST /api/auth/logout
+Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
+```
+
+## Response (204 No Content)
+
+No response body.
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 401 | Invalid or expired token |
+| 403 | Token already invalidated |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->

docs/api/authentication/refresh_token.md

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+# 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 -->

docs/api/authentication/register.md

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+# Register User
+
+Create a new user account.
+
+**Endpoint**: `POST /api/auth/register`
+**Auth**: Not required
+**Content-Type**: `application/json`
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| email | string | Yes | User's email address |
+| username | string | Yes | Desired username |
+| password | string | Yes | Password (min 8 chars) |
+| first_name | string | No | User's first name |
+| last_name | string | No | User's last name |
+
+### Example Request
+
+```json
+{
+  "email": "user@example.com",
+  "username": "john",
+  "password": "SecureP@ss123!",
+  "first_name": "John",
+  "last_name": "Doe"
+}
+```
+
+## Response (201 Created)
+
+```json
+{
+  "token": "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...",
+  "refresh_token": "d4f5g6h7...",
+  "user": {
+    "id": "uuid-here",
+    "email": "user@example.com",
+    "username": "john",
+    "role": "user",
+    "theme": "tokyo-night",
+    "created_at": "2026-01-31T10:00:00Z"
+  }
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid email format, weak password, or missing fields |
+| 409 | Email or username already exists |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->

docs/api/users/change_password.md

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+# 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 -->

docs/api/users/get_profile.md

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+# Get Current User Profile
+
+Retrieve the current authenticated user's profile.
+
+**Endpoint**: `GET /api/users/me`
+**Auth**: Required
+
+## Request Headers
+
+| Header | Type | Required | Description |
+|--------|------|-----------|-------------|
+| Authorization | string | Yes | Bearer token |
+
+### Example Request
+
+```http
+GET /api/users/me
+Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
+```
+
+## Response (200 OK)
+
+```json
+{
+  "id": "uuid",
+  "email": "user@example.com",
+  "username": "john",
+  "first_name": "John",
+  "last_name": "Doe",
+  "theme": "tokyo-night",
+  "role": "user",
+  "max_devices": 10,
+  "created_at": "2026-01-31T10:00:00Z"
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 401 | Invalid or expired token |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->

docs/api/users/update_profile.md

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+# Update Profile
+
+Update the current user's profile information.
+
+**Endpoint**: `PUT /api/users/me/profile`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| first_name | string | No | User's first name |
+| last_name | string | No | User's last name |
+
+### Example Request
+
+```json
+{
+  "first_name": "John",
+  "last_name": "Smith"
+}
+```
+
+## Response (200 OK)
+
+```json
+{
+  "id": "uuid",
+  "email": "user@example.com",
+  "username": "john",
+  "first_name": "John",
+  "last_name": "Smith",
+  "theme": "tokyo-night",
+  "role": "user",
+  "max_devices": 10,
+  "created_at": "2026-01-31T10:00:00Z"
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid input data |
+| 401 | Invalid or expired token |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->

docs/api/users/update_theme.md

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+# Update Theme
+
+Update the current user's theme preference.
+
+**Endpoint**: `PUT /api/users/me/theme`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| theme | string | Yes | Theme name (e.g., "tokyo-night", "dracula") |
+
+### Example Request
+
+```json
+{
+  "theme": "dracula"
+}
+```
+
+## Response (200 OK)
+
+```json
+{
+  "id": "uuid",
+  "email": "user@example.com",
+  "username": "john",
+  "theme": "dracula",
+  "role": "user"
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid theme name |
+| 401 | Invalid or expired token |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->