fix insertMessages: use COUNT(*) instead of MessageRecord.fetchOne for existence check

MessageRecord.fetchOne decodes all columns but the query only selects id,
causing "column not found: accountId". Use COUNT(*) which returns a plain Int.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Packages/MagnumOpusCore/Sources/MailStore/MailStore.swift

@@ -61,12 +61,11 @@ public final class MailStore: Sendable {
 		try dbWriter.write { db in
 			var inserted: [MessageRecord] = []
 			for message in messages {
-				// Check if a message with this (mailboxId, uid) already exists
-				let exists = try MessageRecord.fetchOne(db, sql:
-					"SELECT id FROM message WHERE mailboxId = ? AND uid = ?",
+				let count = try Int.fetchOne(db, sql:
+					"SELECT COUNT(*) FROM message WHERE mailboxId = ? AND uid = ?",
 					arguments: [message.mailboxId, message.uid]
-				)
-				if exists == nil {
+				) ?? 0
+				if count == 0 {
 					try message.insert(db)
 					inserted.append(message)
 				}