reuse add_book_to_title to promote books without title

[elfkuzco]

Changes

• move logic to add_book_to_title to db/book module and reuse when we are applying action to create title. This action is never attached with other actions that restore title or update title metadata because title never exists at that point. This way, when we are adding book to title, book filename is computed from filename along with other helpful events that get logged when book is newly added to title

This fixes #463

[codecov]

Codecov Report

❌ Patch coverage is 72.09302% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.12%. Comparing base (5f8d26a) to head (33cc1ec).

Files with missing lines	Patch %	Lines
backend/src/cms_backend/db/book.py	69.23%	9 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #464      +/-   ##
==========================================
+ Coverage   81.04%   81.12%   +0.08%     
==========================================
  Files          65       64       -1     
  Lines        3883     3879       -4     
  Branches      472      472              
==========================================
  Hits         3147     3147              
+ Misses        583      581       -2     
+ Partials      153      151       -2     

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[benoit74]

LGTM