fix: Fixed media duplication

[frgfm]

This PR introduces the following modifications:

• updates bucket name resolution: formerly hash first 32 characters + ".file", now timestamp + fash first 8 characters + extension (using mimetype guessing)
• delete the previous file from bucket when a media is assigned a new file via upload
• deletes file when media is deleted
• updates test accordingly

Closes #94

[codecov[bot]]

Codecov Report

Merging #207 (283416e) into master (7fdeded) will decrease coverage by 0.02%. The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master     #207      +/-   ##
==========================================
- Coverage   92.60%   92.58%   -0.03%     
==========================================
  Files          34       34              
  Lines        1204     1213       +9     
==========================================
+ Hits         1115     1123       +8     
- Misses         89       90       +1     

Flag	Coverage Δ
unittests	92.58% <90.90%> (-0.03%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/app/api/routes/media.py	83.95% <90.90%> (+0.61%)	:arrow_up:

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