fix(webapp): validate mollifier drain shutdown timeout before starting polling loop

The drainer was started inside the singleton factory, with the
shutdown-timeout-vs-GRACEFUL_SHUTDOWN_TIMEOUT reconciliation living in
worker.server.ts init() afterwards. If that validation threw, the polling
loop was already running and the SIGTERM handler registration below it
was never reached — the loop kept polling with no graceful-shutdown
path, and the singleton was cached in its running state (so subsequent
init() calls returned the same drainer and validation kept failing).

Move the timeout check into initializeMollifierDrainer() before
drainer.start(). singleton() uses ??=, so a throw inside the factory
leaves the cache slot unset and the next getMollifierDrainer() call
re-runs the factory — no half-started state, no missing SIGTERM
handler. The catch in worker.server.ts init() still logs and aborts
drainer registration on either the validation error or a Redis init
failure.

Author: d-cs