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

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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 -->