Add additional handling to schema upgrade

When upgrading the backup database's schema from 3 to 4, one step is to
update any markers we've already added to the Plex database to use epoch
timestamps instead of time strings. However, if another version of
IntroEditor already ran that migration, our update query will fail. Add
some logic that checks whether the migration already ran, and if we
still can't get a valid value, reset it.

Server/MarkerBackupManager.js

@@ -443,8 +443,23 @@ class MarkerBackupManager {
         const modifiedMarkers = await db.all(modifiedMarkersQuery);
         const txn = new TransactionBuilder(db);
         for (const marker of modifiedMarkers) {
-            const userCreated = marker.thumb_url.endsWith('*');
-            const date = (new Date(userCreated ? marker.thumb_url.substring(0, marker.thumb_url.length - 1) : marker.thumb_url).getTime() / 1000) * (userCreated ? -1 : 1);
+            let userCreated = marker.thumb_url.endsWith('*');
+            let date = (new Date(userCreated ? marker.thumb_url.substring(0, marker.thumb_url.length - 1) : marker.thumb_url).getTime() / 1000) * (userCreated ? -1 : 1);
+            if (isNaN(date)) {
+                // Did another instance already switch to epoch?
+                const asEpoch = Math.abs(marker.thumb_url);
+                if (isNaN(asEpoch)) {
+                    // Bad data, reset to blank
+                    date = '';
+                } else {
+                    const year = new Date(asEpoch * 1000).getFullYear();
+                    if (year > 1990 && year < Date.now()) {
+                        date = marker.thumb_url;
+                    } else {
+                        date = '';
+                    }
+                }
+            }
             txn.addStatement(`UPDATE taggings SET thumb_url=? WHERE id=?`, [date, marker.id]);
         }