Both the single-run path and the campaign scheduler drove long-lived
asyncio tasks through their own duplicated _tasks/_cancel_events dicts and
shutdown loops. Collapse them into one deep TaskRegistry module,
instantiated as run_registry and campaign_registry. launch() creates the
cancel event before the task (so a cancel during startup is never lost),
wires done-callback cleanup, and is idempotent per id; this makes runs.py's
hard-cancel fallback provably dead, so it is removed. App shutdown now
gracefully stops in-flight runs too, not just campaigns.
