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/youtube/youtube.go

@@ -234,6 +234,7 @@ func (a *Adapter) NewVideos(ctx context.Context, sub domain.Subscription) ([]dom
 			Provider:        domain.ProviderYouTube,
 			ProviderVideoID: vid,
 			Title:           item.Snippet.Title,
+			ChannelTitle:    sub.ChannelTitle,
 			URL:             "https://www.youtube.com/watch?v=" + vid,
 			PublishedAt:     item.Snippet.PublishedAt,
 		})
@@ -270,6 +271,7 @@ func (a *Adapter) VideoByID(ctx context.Context, userID, videoID string) (domain
 		Provider:        domain.ProviderYouTube,
 		ProviderVideoID: videoID,
 		Title:           it.Snippet.Title,
+		ChannelTitle:    it.Snippet.ChannelTitle,
 		URL:             "https://www.youtube.com/watch?v=" + videoID,
 		PublishedAt:     it.Snippet.PublishedAt,
 	}, nil
@@ -376,8 +378,9 @@ type playlistItemListResponse struct {
 type videoListResponse struct {
 	Items []struct {
 		Snippet struct {
-			Title       string    `json:"title"`
-			PublishedAt time.Time `json:"publishedAt"`
+			Title        string    `json:"title"`
+			ChannelTitle string    `json:"channelTitle"`
+			PublishedAt  time.Time `json:"publishedAt"`
 		} `json:"snippet"`
 	} `json:"items"`
 }