refactor: code review fixes

Bug fixes:
- _extract_cursor: only extract Bottom cursors, preventing Top cursor
  from corrupting pagination state
- _api_request: merge _api_get/_api_post into unified method — POST
  now has rate-limit code 88 retry (was missing)
- fetch_user_likes: add override_base_variables=True

Code quality:
- Extract BEARER_TOKEN and USER_AGENT into constants.py (was duped
  in auth.py and client.py)
- Add user_profile_to_dict/users_to_json for proper UserProfile
  serialization (followers/following JSON output was ad-hoc)
- Refactor 6 CLI write commands via _write_action helper
- Extract _extract_media and _extract_author from _parse_tweet_result
- Update CLI module docstring with all 18 commands

twitter_cli/serialization.py

@@ -1,11 +1,11 @@
-"""Serialization helpers for Tweet models."""
+"""Serialization helpers for Tweet and UserProfile models."""
 
 from __future__ import annotations
 
 import json
 from typing import Any, Dict, Iterable, List, Optional
 
-from .models import Author, Metrics, Tweet, TweetMedia
+from .models import Author, Metrics, Tweet, TweetMedia, UserProfile
 
 
 def tweet_to_dict(tweet: Tweet) -> Dict[str, Any]:
@@ -129,6 +129,34 @@ def tweets_to_json(tweets: Iterable[Tweet]) -> str:
     return json.dumps([tweet_to_dict(tweet) for tweet in tweets], ensure_ascii=False, indent=2)
 
 
+def user_profile_to_dict(user: UserProfile) -> Dict[str, Any]:
+    """Convert a UserProfile dataclass into a JSON-safe dict."""
+    return {
+        "id": user.id,
+        "name": user.name,
+        "screenName": user.screen_name,
+        "bio": user.bio,
+        "location": user.location,
+        "url": user.url,
+        "followers": user.followers_count,
+        "following": user.following_count,
+        "tweets": user.tweets_count,
+        "likes": user.likes_count,
+        "verified": user.verified,
+        "profileImageUrl": user.profile_image_url,
+        "createdAt": user.created_at,
+    }
+
+
+def users_to_json(users: Iterable[UserProfile]) -> str:
+    """Serialize UserProfile objects to pretty JSON."""
+    return json.dumps(
+        [user_profile_to_dict(user) for user in users],
+        ensure_ascii=False,
+        indent=2,
+    )
+
+
 def _optional_int(value: Any) -> Optional[int]:
     """Parse an optional integer value."""
     if value is None: