feat(detections): restrict webhook and telegram to sequence creation

[frgfm]

As mentioned in #413, I think the webhook trigger and Telegram notifications should only be considered when a sequence is created (several detections in a row from the same camera and azimuth) vs. when a detection is created (more prone to false positives and noise).

This PR moves both to sequence creation

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.

Project coverage is 85.23%. Comparing base (6c8b02f) to head (bd2c7e8).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/app/api/api_v1/endpoints/detections.py	0.00%	9 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #416   +/-   ##
=======================================
  Coverage   85.23%   85.23%           
=======================================
  Files          38       38           
  Lines        1097     1097           
=======================================
  Hits          935      935           
  Misses        162      162           

Flag	Coverage Δ
backend	84.57% <0.00%> (ø)
client	93.67% <ø> (ø)

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