Fix RFC3339 datetimes and ignore empty favourites
Add a SQL migration (20260501000000_fix_datetime_formats.sql) that:
- sets NULL/empty created_at/updated_at to the current time
- converts "YYYY-MM-DD HH:MM:SS" timestamps to RFC3339 so SQLx can
decode DateTime<Utc>
Execute the migration during pool initialization.
Tighten favourites queries to use INNER JOIN and filter out empty-
///string IDs so empty/bogus favourite entries are excluded.