feat(webhooks): add webhook mechanism on detection creation

[frgfm]

This PR adds a small webhook table, and used when detection are created as background tasks. I made webhooks editable only by admins, and they post the detection payload for now. We'll be able to edit what gets sent upon dispatch later if needed

Here is the new UML:

[codecov]

Codecov Report

Attention: Patch coverage is 82.85714% with 12 lines in your changes missing coverage. Please review.

Project coverage is 86.19%. Comparing base (92b9e28) to head (bd58f3d).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/app/api/dependencies.py	53.33%	7 Missing ⚠️
src/app/api/api_v1/endpoints/detections.py	44.44%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #367      +/-   ##
==========================================
- Coverage   86.58%   86.19%   -0.39%     
==========================================
  Files          31       34       +3     
  Lines         857      920      +63     
==========================================
+ Hits          742      793      +51     
- Misses        115      127      +12     

Flag	Coverage Δ
backend	85.79% <82.85%> (-0.39%)	⬇️
client	91.80% <ø> (ø)

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