Two trigger paths land here:
- A 30s ticker in cmd/server calls Server.DrainAllDue(ctx). It
walks pending_runs rows whose next_attempt_at <= now, dedupes by
host, skips offline hosts, and per online host runs DrainPending.
- onAgentHello spawns a background DrainPending(hostID). When a
host comes back, every pending row for it is dispatchable now —
due-ness becomes irrelevant once the wire is back.
Each row's schedule + group are reloaded; ErrNotFound or
disabled-schedule or gone-group abandons the row with a
pending_run.abandoned audit. attempt >= retry_max also abandons.
Otherwise dispatchBackupForGroup is invoked; success deletes the
row, failure bumps attempt with exponential backoff capped at
30m.
