feat(client): per-call timeout for heartbeat

[MateoLostanlen]

• The engine sends a heartbeat on every frame and must not hold its loop for the client-wide 10 s timeout, which image uploads still need.
• Client.heartbeat() gains an optional timeout argument used for that request only; omitted, it keeps the client timeout, so existing callers are unchanged.
• Lets pyro-engine drop its SIGALRM-based heartbeat cutoff, which deadlocked urllib3 on kwartier-west on 2026-09-11.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.87%. Comparing base (c203470) to head (361d7a6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #691      +/-   ##
==========================================
+ Coverage   91.30%   93.87%   +2.56%     
==========================================
  Files           3       59      +56     
  Lines         138     3214    +3076     
==========================================
+ Hits          126     3017    +2891     
- Misses         12      197     +185     

Flag	Coverage Δ
backend	93.98% <ø> (?)
client	91.30% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.