Improve migration performance

[codingjoe]

• perform delete IO async
• create only newly added sizes
• delete only obsolete file sizes
• create sizes from an in-memory copy of the file

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (f361b9c) to head (0a676e2).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #169   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines          466       488   +22     
=========================================
+ Hits           466       488   +22     

Flag	Coverage Δ
celery	84.42% <90.41%> (+0.94%)	:arrow_up:
cleanup	87.90% <91.78%> (+3.78%)	:arrow_up:
dj4.1	87.90% <91.78%> (+3.78%)	:arrow_up:
dj4.2	87.90% <91.78%> (+3.78%)	:arrow_up:
django-rq	84.42% <90.41%> (+0.94%)	:arrow_up:
dramatiq	84.42% <90.41%> (+0.94%)	:arrow_up:
drf	96.31% <91.78%> (+3.39%)	:arrow_up:
macos-latest	88.11% <91.78%> (+3.77%)	:arrow_up:
py3.10	87.90% <91.78%> (+3.78%)	:arrow_up:
py3.12	87.90% <91.78%> (+3.78%)	:arrow_up:
py3.8	87.90% <91.78%> (+3.78%)	:arrow_up:
py3.9	87.90% <91.78%> (+3.78%)	:arrow_up:
ubuntu-latest	87.90% <91.78%> (+3.78%)	:arrow_up:
windows-latest	81.01% <86.66%> (+5.20%)	:arrow_up:

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