perf(feed): index the default published_at DESC NULLS LAST sort (E2 S1, PERF-1)

The feed's default `newest` sort is `published_at DESC NULLS LAST, id ASC`, but
the only published_at index was a plain ASC btree — read backwards it satisfies
DESC NULLS FIRST, not NULLS LAST — so every feed page fell back to a full
`videos` seq scan + top-N sort. Migration 0067 adds the two matching indexes
(global + per-channel) CONCURRENTLY and drops the now-redundant single-column
ix_videos_channel_id (the channel composite leads with channel_id, a strict
superset).

Measured on the prod-sized localdev reseed (257,312 videos, prod planner):
- global newest:  44,747 buffers / 154 ms  ->  60 buffers / 0.32 ms
- per-channel(12.6k): 7,604 buffers / 96 ms -> 54 buffers / 0.14 ms
CONCURRENTLY build of both on 257k rows: 1.7s total (~0 downtime). This is the
likely root cause of the "prod feed is slow" symptom.

Model: drop the inline index=True on Video.channel_id (the composite supersedes
it); the composites are migration-defined like the search_vector GIN. Gate green
(334 pytest incl. the migration drift + downgrade/re-upgrade round-trip).

backend/app/models.py

@@ -251,8 +251,12 @@ class Video(Base, TimestampMixin):
     __tablename__ = "videos"
 
     id: Mapped[str] = mapped_column(String(16), primary_key=True)  # video id
+    # No standalone channel_id index: the composite `ix_videos_channel_pub_desc`
+    # (channel_id, published_at DESC NULLS LAST, id) from migration 0067 leads with channel_id, so it
+    # serves every channel_id lookup (FK cascade, sync upsert, channel-detail) AND the per-channel feed
+    # sort. Like the search_vector GIN, this composite is migration-defined, not an inline `index=True`.
     channel_id: Mapped[str] = mapped_column(
-        ForeignKey("channels.id", ondelete="CASCADE"), index=True
+        ForeignKey("channels.id", ondelete="CASCADE")
     )
     title: Mapped[str | None] = mapped_column(Text)  # normalized for display (see app.titles)
     original_title: Mapped[str | None] = mapped_column(Text)  # raw YouTube title, preserved