fix(shared): filter due proposals by source_platform to prevent cross-platform race

Both Telegram and Discord deadline services were querying ALL due
proposals without filtering by source_platform. If the Telegram
service reached a Discord proposal first, it finalized the DB state
but skipped message handling. The Discord service then saw status
!= 'Voting' and never updated the Discord vote message.

Fix: GetDueProposalIdsAsync now accepts a sourcePlatform parameter
and filters at the DB level. Each service only processes its own
platform's proposals.

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

src/GmRelay.DiscordBot/Features/Sessions/DiscordRescheduleVotingDeadlineService.cs

@@ -34,7 +34,7 @@ public sealed class DiscordRescheduleVotingDeadlineService(
     {
         try
         {
-            var proposalIds = await finalizer.GetDueProposalIdsAsync(ct);
+            var proposalIds = await finalizer.GetDueProposalIdsAsync("Discord", ct);
             foreach (var id in proposalIds)
             {
                 await TryFinalizeAsync(id, ct);