Yesterday's scheduler-died incident (#85) was one instance of a systemic
bug: every long-running goroutine in the platform lacks panic recovery
and exposes no liveness signal. In a multi-tenant SaaS deployment, a
single tenant's bad data panicking any subsystem takes down the
subsystem for every tenant, silently, with all standard health probes
still green. That is a scale-of-one sev-1.
This PR:
1. Introduces `platform/internal/supervised/` with two primitives:
a. RunWithRecover(ctx, name, fn) — runs fn in a recover wrapper.
On panic logs the stack + exponential-backoff restart (1s → 2s →
4s → … → 30s cap). On clean return (fn decided to stop) returns.
On ctx.Done cancels cleanly.
b. Heartbeat(name) + LastTick(name) + Snapshot() + IsHealthy(names,
staleThreshold) — shared in-memory liveness registry. Every
subsystem calls Heartbeat(name) at the end of each tick so
operators can distinguish "goroutine alive and healthy" from
"alive but stuck inside a single tick".
2. Wraps every `go X.Start(ctx)` in main.go:
- broadcaster.Subscribe (Redis pub/sub relay → WebSocket)
- registry.StartLivenessMonitor
- registry.StartHealthSweep
- scheduler.Start (the one that died yesterday)
- channelMgr.Start (Telegram / Slack)
3. Adds `supervised.Heartbeat("scheduler")` inside the scheduler tick
loop as the first end-to-end demonstration. Follow-up PRs will add
heartbeats to the other four subsystems.
4. Adds `GET /admin/liveness` endpoint returning per-subsystem
last_tick_at + seconds_ago. Operators can poll this and alert on
any subsystem whose seconds_ago exceeds 2x its cron/tick interval.
5. Unit tests for RunWithRecover (clean return no restart; panic
restarts with backoff; ctx cancel stops restart loop) and for the
liveness registry.
Net new code: ~160 lines + ~100 lines of tests. Refactor of main.go:
~10 line changes. No behavior change on happy path; only lifts what
happens on a panic.
Closes#92. Supersedes the local recover added to scheduler.go in
#90 (kept conceptually, but now via the shared helper).
