collections: listing answers channel and download queries from covering indexes (#740)

The listing still read every channel row (for the summary columns) and every download
row of the listed Collections (for the minimum frequency). Both tables store yt-dlp
info_json, so rows are tens of KB and each sits on its own scattered page; selecting
fewer columns did not help because the seek per row is the cost. From a cold disk
cache that was 5 s per table, 10+ s total, on a large library.

Two covering indexes, channel (collection_id, video_count, total_size) and
download (collection_id, frequency), answer both queries without touching a row. The
listing no longer loads Download objects at all; it computes min_download_frequency
with one grouped query and drops the unused `downloads` array from listing rows (the
detail endpoints keep it; the list views only read min_download_frequency).

Adds a models-vs-migrations test (alembic check) so an index or column added to a
model without a migration, or vice versa, fails the suite.

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

modules/videos/models.py

@@ -593,6 +593,8 @@ class Channel(ModelHelper, Base):
     __tablename__ = 'channel'
     __table_args__ = (
         Index('channel_minimum_frequency_idx', 'minimum_frequency'),
+        # Covers the Collection listing's summary query; channel rows are wide (info_json).
+        Index('channel_collection_summary_idx', 'collection_id', 'video_count', 'total_size'),
         Index('channel_source_id', 'source_id'),
         Index('channel_total_size_idx', 'total_size'),
         Index('channel_video_count_idx', 'video_count'),