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/videos_test.go

@@ -113,3 +113,29 @@ func TestNewestUnsummarizedVideoIDs(t *testing.T) {
 	require.NoError(t, err)
 	require.Empty(t, none, "limit 0 returns nothing")
 }
+
+func TestUpsertVideoPersistsChannelAndDistinctChannels(t *testing.T) {
+	ctx := context.Background()
+	s := newStore(t)
+	resetDB(t, rawPool(t))
+
+	mk := func(pid, channel string) {
+		v := ytVideo(userA, pid, pid)
+		v.ChannelTitle = channel
+		_, err := s.UpsertVideo(ctx, v)
+		require.NoError(t, err)
+	}
+	mk("aa11111aaaa", "Acme Talks")
+	mk("bb22222bbbb", "Acme Talks") // same channel
+	mk("cc33333cccc", "Zeta Channel")
+	// userB's channel must not leak.
+	vb := ytVideo(userB, "dd44444dddd", "x")
+	vb.ChannelTitle = "Bravo Only"
+	_, err := s.UpsertVideo(ctx, vb)
+	require.NoError(t, err)
+
+	got, err := s.DistinctChannels(ctx, userA)
+	require.NoError(t, err)
+	require.Equal(t, []string{"Acme Talks", "Zeta Channel"}, got,
+		"distinct, alphabetical, user-scoped (no Bravo Only)")
+}