Rework backend to isolate the mill and the shuttle

[benoit74]

Split the backend in 3 components:

• the API
• the mill, responsible for background tasks execution
• the shuttle, responsible to move files around on warehouses

[codecov]

Codecov Report

❌ Patch coverage is 14.44444% with 77 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.37%. Comparing base (6bf6032) to head (0f6d919).
⚠️ Report is 3 commits behind head on cms-upgrade.

Files with missing lines	Patch %	Lines
backend/src/cms_backend/mill/main.py	0.00%	27 Missing ⚠️
backend/src/cms_backend/shuttle/main.py	0.00%	26 Missing ⚠️
backend/src/cms_backend/mill/context.py	0.00%	10 Missing ⚠️
backend/src/cms_backend/shuttle/context.py	0.00%	10 Missing ⚠️
.../cms_backend/mill/process_zimfarm_notifications.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           cms-upgrade     #125      +/-   ##
===============================================
- Coverage        85.20%   78.37%   -6.84%     
===============================================
  Files               28       33       +5     
  Lines              926     1008      +82     
  Branches            80       86       +6     
===============================================
+ Hits               789      790       +1     
- Misses             117      198      +81     
  Partials            20       20              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.