migration: sync catchup_provider_stream_id help_text

The Channel.catchup_provider_stream_id model field carries a help_text that
was not reflected in migration 0038, leaving makemigrations --check dirty.
Add the AlterField migration so model and migration state match (no schema
change — help_text only).

apps/channels/migrations/0039_alter_channel_catchup_provider_stream_id.py

@@ -0,0 +1,18 @@
+# Generated by Django 6.0.5 on 2026-06-01 10:57
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('dispatcharr_channels', '0038_add_catchup_fields'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='channel',
+            name='catchup_provider_stream_id',
+            field=models.CharField(blank=True, default='', help_text='Provider stream_id of the highest-priority catch-up stream (for timeshift URL construction)', max_length=64),
+        ),
+    ]