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_test.go

@@ -131,7 +131,7 @@ func TestNewVideos(t *testing.T) {
 		}`))
 	})
 
-	sub := domain.Subscription{ID: "s1", UserID: "u1", ChannelID: "UC_acme"}
+	sub := domain.Subscription{ID: "s1", UserID: "u1", ChannelID: "UC_acme", ChannelTitle: "Acme Channel"}
 	vids, err := a.NewVideos(context.Background(), sub)
 	if err != nil {
 		t.Fatalf("NewVideos: %v", err)
@@ -143,6 +143,9 @@ func TestNewVideos(t *testing.T) {
 	if v.ProviderVideoID != "vid1" || v.Title != "Designing for Attention" {
 		t.Errorf("unexpected video: %+v", v)
 	}
+	if v.ChannelTitle != "Acme Channel" {
+		t.Errorf("ChannelTitle = %q, want Acme Channel", v.ChannelTitle)
+	}
 	if v.Provider != domain.ProviderYouTube || v.URL != "https://www.youtube.com/watch?v=vid1" {
 		t.Errorf("video not wired correctly: %+v", v)
 	}