Correctly handle cases where book location and/or filename are already correct

[benoit74]

If the book already has a correct filename, we do not want to create a move operation nor compute a useless new name (e.g. move tests-bbe_fr_all_2025-03.zim to tests-bbe_fr_all_2025-03a.zim)

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.20%. Comparing base (41f3c80) to head (6bf6032).
⚠️ Report is 3 commits behind head on cms-upgrade.

Files with missing lines	Patch %	Lines
backend/src/cms_backend/processors/book.py	80.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           cms-upgrade     #124      +/-   ##
===============================================
- Coverage        85.68%   85.20%   -0.48%     
===============================================
  Files               28       28              
  Lines              908      926      +18     
  Branches            76       80       +4     
===============================================
+ Hits               778      789      +11     
- Misses             114      117       +3     
- Partials            16       20       +4     

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