fix(#20): reset confirmation_sent_at on reschedule and add guard

- RescheduleVotingDeadlineService: clear confirmation_sent_at +
  confirmation_message_id when moving session back to Planned.
- HandleRescheduleTimeInputHandler.RescheduleImmediately: same reset.
- SendConfirmationHandler: add confirmation_sent_at IS NULL guard
  to prevent duplicate confirmation messages if DB update fails.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/GmRelay.Bot/Features/Confirmation/SendConfirmation/SendConfirmationHandler.cs

@@ -112,6 +112,7 @@ public sealed class SendConfirmationHandler(
                 confirmation_sent_at = now(),
                 updated_at = now()
             WHERE id = @SessionId
+              AND confirmation_sent_at IS NULL
             """,
             new
             {