feat: add author follower and following counts to serialized output

[edwinbernadus]

Summary

Expose author follower/following counts in parsed tweet output.

This adds followers_count and following_count to the Author model,
parses them
from Twitter user legacy fields (followers_count and friends_count), and
includes
them in serialized tweet output as author.followers and author.following.

Changes

File	Change
twitter_cli/models.py	Added followers_count and following_count fields to the Author dataclass.
twitter_cli/parser.py	Parse followers_count and friends_count from user_legacy into the Author model.
twitter_cli/serialization.py	Include author.followers and author.following in tweet_to_dict() output.

Impact

Area	Before	After
Author model	No follower/following counts	Includes followers_count and following_count
Parsed author metadata	Name, screen name, avatar, verified	Adds follower and following counts
Structured output	No author.followers / author.following	Both fields available in serialized output

Notes

• followers_count is sourced from Twitter's user_legacy.followers_count
• following_count is sourced from Twitter's user_legacy.friends_count
• Default value is 0 when the source field is missing or unparsable