fix: keep alert id stable when labeling a solo sequence FP

[MateoLostanlen]

• When a sequence labeled non-wildfire is the only one in its alert, the previous flow detached it, deleted the (now empty) alert, and created an identical replacement — churning the alert id for no reason.
• Now the solo case is detected via a LIMIT 1 siblings probe and the alert is left untouched.
• Multi-sequence alerts still split out the FP into its own alert so it stops contributing to triangulation.
• Adds an integration test for the stable-id case and a unit test for the early-return branch; updates the existing multi-sequence unit test to mock the new probe.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.82%. Comparing base (b4fa415) to head (2bdc507).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #584      +/-   ##
==========================================
+ Coverage   89.80%   89.82%   +0.02%     
==========================================
  Files          55       55              
  Lines        2422     2427       +5     
==========================================
+ Hits         2175     2180       +5     
  Misses        247      247              

Flag	Coverage Δ
backend	89.79% <100.00%> (+0.02%)	⬆️
client	90.40% <ø> (ø)

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