Add user profile endpoint (#541)

* feat: Implement UserProfile serializer and add API endpoint per #457

* chore: Document API addition

* Address review comments

---------

Co-authored-by: fkulla <mail@florian.direct>

docs/API.md

@@ -236,3 +236,35 @@ Example payload:
   "name": "example"
 }
 ```
+
+### User
+
+**Profile**
+
+```
+GET /api/user/profile/
+```
+
+User preferences.
+
+Example response:
+
+```json
+{
+  "theme": "auto",
+  "bookmark_date_display": "relative",
+  "bookmark_link_target": "_blank",
+  "web_archive_integration": "enabled",
+  "tag_search": "lax",
+  "enable_sharing": true,
+  "enable_public_sharing": true,
+  "enable_favicons": false,
+  "display_url": false,
+  "permanent_notes": false,
+  "search_preferences": {
+    "sort": "title_asc",
+    "shared": "off",
+    "unread": "off"
+  }
+}
+```