fix: reject unavailable frames and pace health checks

[hoangsvit]

Summary

Corrects the scheduled camera health checker already on main.

Fixes

• detect the upstream static IMAGE NOT AVAILABLE payload as unavailable even though the proxy returns HTTP 200 + image/jpeg
• detection uses the actual PNG signature and stable 290x183 dimensions observed from live probes
• reduce health-check concurrency from 12 to 2
• add 250ms pacing between checks per worker
• increase workflow timeout to 20 minutes to allow safer low-rate scanning

Why

A concurrency-12 run caused the upstream to return the unavailable placeholder for 634/797 cameras, which polluted Online/Offline totals. Lowering request pressure prevents the checker from creating the failure condition it is trying to measure.

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