fix(engine): drop the SIGALRM heartbeat cutoff that can deadlock the loop

[MateoLostanlen]

• On 2026-09-11 the kwartier-west engine froze for 26 h with Docker still reporting it healthy. py-spy showed the main thread stuck on a urllib3 pool lock: the heartbeat's SIGALRM raised inside the HTTP call while the lock was held, and the session finalizer then blocked on it forever.
• The heartbeat now uses a plain 3 s requests timeout via the new Client.heartbeat(timeout=...) from feat(client): per-call timeout for heartbeat pyro-api#691, so a slow API is dropped with a warning instead of interrupting urllib3 mid-call.
• The engine healthcheck now checks that engine.log moved in the last 70 min and kills the process when it did not, so restart: always brings a hung engine back.
• pyroclient is pinned to the PR 691 commit; to switch back to @main and re-lock once it merges.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.35%. Comparing base (2465419) to head (a38b545).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #419      +/-   ##
===========================================
+ Coverage    79.11%   79.35%   +0.23%     
===========================================
  Files            6        6              
  Lines          838      833       -5     
===========================================
- Hits           663      661       -2     
+ Misses         175      172       -3     

Flag	Coverage Δ
unittests	79.35% <100.00%> (+0.23%)	⬆️

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