The #95 scheduler heartbeat scheme relied on:
1. Top of tick() (once per poll interval)
2. Per-fire goroutine entry + exit
That leaves a gap: tick() ends with wg.Wait(), so if a single fire takes
longer than pollInterval (UIUX audits routinely take 60-120s; max fireTimeout
is 5min), the next tick doesn't run and no top-of-tick heartbeat fires.
Per-fire heartbeats only bracket the fire — between entry and the HTTP
response returning, nothing heartbeats either.
Observed today: /admin/liveness reports seconds_ago=251 while docker logs
show the scheduler actively firing 'Hourly ecosystem watch'. Scheduler is
fine; liveness is lying.
Adds an independent 10s heartbeat pulse goroutine inside Start(), decoupled
from tick completion. The existing heartbeats at tick top + per-fire are
kept as redundant signals but this pulse is the one that guarantees liveness
freshness regardless of what tick is doing.
Ships the exact fix proposed in #140 body.
Closes#140.
b00f478b6e