Enhance title merging logic to handle additional episode part indicators (#409)

[tsnearly]

Fixes #409

Changes

• Created merge_titles function, ensuring merged episode titles and sort-titles are cleaner if applicable.
• Introduced MERGE_TITLE_PATTERN to match various part indicators in episode titles.
• The merging process now extracts a base title from the first matching episode and appends it appropriately, avoiding duplicate or confusing titles in the merged result.

Impact

• Merged episodes will have cleaner titles if they are continuations.
• Added dependency on module re

[tsnearly]

This PR improves title/summary merging logic and episode group handling, adds clarity, and makes the script more robust. The new merge_titles function provides more intelligent title merging for multi-episode cases, and general code quality is improved.