fix FK constraint: skip duplicate messages before thread reconstruction

- insertMessages checks (mailboxId, uid) existence before INSERT, returns
  only actually inserted records
- syncMailbox only runs ThreadReconstructor on newly inserted messages,
  preventing FK violations from stale UUIDs referencing ignored records
- improve error logging to show full error description

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

Apps/MagnumOpus/ViewModels/MailViewModel.swift

@@ -207,10 +207,10 @@ final class MailViewModel {
 			try await coordinator.syncNow()
 			syncState = coordinator.syncState
 		} catch {
-			let desc = "\(type(of: error)): \(error.localizedDescription)"
+			let desc = "\(error)"
 			print("[MailViewModel] syncNow failed: \(desc)")
-			errorMessage = desc
-			syncState = .error(desc)
+			errorMessage = error.localizedDescription
+			syncState = .error(error.localizedDescription)
 		}
 	}
 

Packages/MagnumOpusCore/Sources/MailStore/MailStore.swift

@@ -55,12 +55,23 @@ public final class MailStore: Sendable {
 
 	// MARK: - Messages
 
-	public func insertMessages(_ messages: [MessageRecord]) throws {
+	/// Insert messages, skipping duplicates by (mailboxId, uid). Returns only the records that were actually inserted.
+	@discardableResult
+	public func insertMessages(_ messages: [MessageRecord]) throws -> [MessageRecord] {
 		try dbWriter.write { db in
+			var inserted: [MessageRecord] = []
 			for message in messages {
-				// INSERT OR IGNORE: skip if (mailboxId, uid) already exists from a prior sync
-				try message.insert(db, onConflict: .ignore)
+				// 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 = ?",
+					arguments: [message.mailboxId, message.uid]
+				)
+				if exists == nil {
+					try message.insert(db)
+					inserted.append(message)
+				}
 			}
+			return inserted
 		}
 	}
 

Packages/MagnumOpusCore/Sources/SyncEngine/SyncCoordinator.swift

@@ -163,12 +163,13 @@ public final class SyncCoordinator {
 			let records = envelopes.map { envelope -> MessageRecord in
 				envelopeToRecord(envelope, accountId: accountConfig.id, mailboxId: mailboxId)
 			}
-			try store.insertMessages(records)
+			let inserted = try store.insertMessages(records)
 
-			let reconstructor = ThreadReconstructor(store: store)
-			try reconstructor.processMessages(records)
-
-			emit(.newMessages(count: envelopes.count, mailbox: remoteMailbox.name))
+			if !inserted.isEmpty {
+				let reconstructor = ThreadReconstructor(store: store)
+				try reconstructor.processMessages(inserted)
+				emit(.newMessages(count: inserted.count, mailbox: remoteMailbox.name))
+			}
 		}
 
 		// Reconcile flags for recent existing messages (read/unread state from other devices)