[books] Allow timezone changes when importing from KOReader

Turns out you need a city-based timezone for DST stuff to work properly.
The US/Eastern timezone doesn't mess with DST because it can be so wonky
in different regions. So while we fix timezone defaulting to a
DST-friendly timezone too.

vrobbler/apps/profiles/migrations/0026_userprofile_timezone_change_log.py

@@ -0,0 +1,18 @@
+# Generated by Django 4.2.19 on 2025-07-11 22:24
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('profiles', '0025_rename_bgstat_id_userprofile_bgstats_id'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='userprofile',
+            name='timezone_change_log',
+            field=models.TextField(blank=True, null=True),
+        ),
+    ]