15 minutes before a posted op starts, the bot opens a thread on the
roll-call message and pings everyone who RSVP'd yes, as a last-call
nudge that the op is imminent. One-shot per mission via a new
imminentReminderSentAt marker on Missions (migration included); the
cancel flow clears it so a rescheduled op re-reminds for its new start
time. Poll cadence via DISCORD_IMMINENT_POLL_MS (default 60s).
