Add three new named health checks to the manifest alongside the existing
web check, each implemented as a fetch+parse of /api/health:
ckpool — is the stratum server alive? (miners can connect)
bitcoin — is Bitcoin Core RPC reachable? (blocks can be submitted)
submit-gap — have all block submissions been confirmed by bitcoind?
The web check retains its current role: calling /api/health which returns
503 when any critical subsystem is down, so StartOS marks the overall
service as degraded. The three new checks give operators a drill-down
view of which subsystem is the problem.
Each check returns "starting" for the first 20 s after launch so normal
boot sequencing does not trigger false alarms.