feat(sequences): re-run cone matching when relabeling back to wildfire_smoke

[MateoLostanlen]

• Context: when a sequence is mis-labeled as non-wildfire (e.g. other_smoke), it's detached from its triangulating alert and a fresh single-sequence alert is created. Reverting the label to wildfire_smoke previously left the sequence stuck in that lonely alert, breaking triangulation. This PR makes the revert path drop the lonely alert and re-run cone matching so the sequence merges back with its overlapping neighbours.
• Anchor the candidate-sequence window on the sequence's own last_seen_at (not utcnow()) so relabels done hours later still find the original neighbours within SEQUENCE_RELAXATION_SECONDS.
• Rename _attach_sequence_to_alert → attach_sequence_to_alert and extract _detach_sequence_from_alerts to deduplicate the detach/refresh block.

[codecov]

Codecov Report

❌ Patch coverage is 95.23810% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.80%. Comparing base (b4fa415) to head (bb5893b).

Files with missing lines	Patch %	Lines
src/app/api/api_v1/endpoints/sequences.py	94.11%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #585   +/-   ##
=======================================
  Coverage   89.80%   89.80%           
=======================================
  Files          55       55           
  Lines        2422     2432   +10     
=======================================
+ Hits         2175     2184    +9     
- Misses        247      248    +1     

Flag	Coverage Δ
backend	89.77% <95.23%> (+<0.01%)	⬆️
client	90.40% <ø> (ø)

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