fix(migrations): make air_timezone column add idempotent

Use ADD COLUMN IF NOT EXISTS so re-running 162 on a database that already has
the column is a no-op.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

migrations/162_media_items_air_timezone.up.sql

@@ -1 +1 @@
-ALTER TABLE media_items ADD COLUMN air_timezone text;
+ALTER TABLE media_items ADD COLUMN IF NOT EXISTS air_timezone text;