perf(gmail): batch the duplicate-message check in CreateSuggestedJob

CreateSuggestedJob ran one AnyAsync per message in the thread to decide
imported-vs-skip — an N+1 that scales with thread length. Replace it with a
single query that loads the already-imported ExternalMessageIds for the job,
then check in memory (identical skip/import behaviour), mirroring the batched
pattern RelinkThread already uses.

Build clean; backend suite 135/135 green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

JobTrackerApi/Controllers/GmailController.cs

@@ -641,12 +641,17 @@ public sealed class GmailController : ControllerBase
 
         var threadMessages = await _gmail.ListThreadMessagesAsync(ownerUserId, request.ThreadId.Trim(), cancellationToken);
         var distinctMessageIds = threadMessages.Select(message => message.Id).Where(static id => !string.IsNullOrWhiteSpace(id)).Distinct(StringComparer.Ordinal).ToList();
+        // Batch the "already imported?" check with a single query instead of one
+        // AnyAsync per message (N+1), mirroring RelinkThread below.
+        var existingMessageIds = await _db.Correspondences
+            .Where(message => message.JobApplicationId == job.Id && message.ExternalMessageId != null && distinctMessageIds.Contains(message.ExternalMessageId))
+            .Select(message => message.ExternalMessageId!)
+            .ToListAsync(cancellationToken);
         var imported = 0;
         var skipped = 0;
         foreach (var messageId in distinctMessageIds)
         {
-            var existing = await _db.Correspondences.AnyAsync(message => message.JobApplicationId == job.Id && message.ExternalMessageId == messageId, cancellationToken);
-            if (existing)
+            if (existingMessageIds.Contains(messageId, StringComparer.Ordinal))
             {
                 skipped++;
                 continue;