feat(web): real channel filter — multi-select of the user's channels

The free-text 'channel' filter was dead: it exact-matched SummaryRow.Channel,
which is just the provider ('youtube'), because videos never stored their source
channel. Now they do.

- migration 014: videos.channel_title (nullable; existing rows backfill on the
  next discovery pass, pasted videos immediately).
- discovery (NewVideos) + paste (VideoByID) populate channel_title; UpsertVideo
  persists it, preserving an existing title when an update arrives empty.
- store.DistinctChannels lists a user's channels (RLS-scoped); SummaryRow carries
  ChannelTitle via the shared projection.
- Filter: single Channel -> Channels []string, matching on ChannelTitle; the feed
  renders a multi-select of DistinctChannels (hidden until channels exist).
- migrate tests: 014 reversibility + fixed the relative-step counts in the 010/011
  up/down tests (014 shifted the topology).

TDD throughout: channel persist + distinct, adapter channel wiring, multi-channel
filter match, handler channel filter, migration up/down.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/adapters/store/reads.go

@@ -29,6 +29,7 @@ type SummaryRow struct {
 	ProviderVideoID string    // videos.provider_video_id; empty when no videos row
 	Title           string    // videos.title; empty when no videos row
 	Channel         string    // videos.provider for now; empty when no videos row
+	ChannelTitle    string    // videos.channel_title; the source channel, for display + filtering
 	URL             string    // videos.url; empty when no videos row
 	PublishedAt     time.Time // videos.published_at; zero when absent
 	Summary         string
@@ -137,7 +138,8 @@ const selectVideo = `
 	       COALESCE(s.created_at, v.seen_at),
 	       (s.id IS NOT NULL) AS summarized,
 	       v.summarize_requested,
-	       COALESCE(v.transcript_status, '')
+	       COALESCE(v.transcript_status, ''),
+	       COALESCE(v.channel_title, '')
 	  FROM videos v
 	  LEFT JOIN summaries s ON s.video_id = v.id AND s.user_id = v.user_id`
 
@@ -254,6 +256,7 @@ func scanVideoRow(rows pgx.Row) (SummaryRow, error) {
 		&row.Summarized,
 		&row.SummarizeRequested,
 		&row.TranscriptStatus,
+		&row.ChannelTitle,
 	); err != nil {
 		return SummaryRow{}, fmt.Errorf("store: scan video: %w", err)
 	}