add endpoint to retrieve zim download/view urls

[elfkuzco]

Rationale

This PR adds an endpoint to retrieve a download/view URLs for zims

Changes

• add download_base_url and view_base_url to collection model
• add function to query db for zims and compute their download/view urls

This closes #137

[codecov]

Codecov Report

❌ Patch coverage is 91.89189% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.51%. Comparing base (1d0cedc) to head (994f9e6).
⚠️ Report is 3 commits behind head on cms-upgrade.

Files with missing lines	Patch %	Lines
backend/src/cms_backend/db/books.py	88.88%	0 Missing and 2 partials ⚠️
backend/src/cms_backend/api/routes/books.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           cms-upgrade     #154      +/-   ##
===============================================
+ Coverage        80.31%   80.51%   +0.20%     
===============================================
  Files               43       44       +1     
  Lines             1702     1735      +33     
  Branches           149      153       +4     
===============================================
+ Hits              1367     1397      +30     
- Misses             291      292       +1     
- Partials            44       46       +2     

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[benoit74]

LGTM