feat(engine): one heartbeat per camera per minute

[MateoLostanlen]

• Heartbeats were sent on every pose of every loop, 8 per minute for a 4-pose camera. Fleet-wide that is 7.5 requests per second and 89% of the alert API traffic, for a liveness signal the platform only evaluates with a 30 minute threshold.
• The engine now sends at most one heartbeat per camera per heartbeat_period (default 60 s), tracked per camera IP on a monotonic clock. Periodic image uploads and mask fetches are unchanged.
• Cuts heartbeat traffic by roughly 8x with no API or platform change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.24%. Comparing base (2465419) to head (1e33d46).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #420      +/-   ##
===========================================
+ Coverage    79.11%   79.24%   +0.12%     
===========================================
  Files            6        6              
  Lines          838      843       +5     
===========================================
+ Hits           663      668       +5     
  Misses         175      175              

Flag	Coverage Δ
unittests	79.24% <100.00%> (+0.12%)	⬆️

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