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/migrate_test.go

@@ -53,7 +53,9 @@ func TestMigration010LoginEventsUpDown(t *testing.T) {
 	require.True(t, loginEventsExists(t), "login_events must exist at latest migration")
 
 	m := fileMigrator(t)
-	// 011, 012, 013 sit above 010; step them down first so 010 is exercised in isolation.
+	// 011, 012, 013, 014 sit above 010; step them down first so 010 is exercised in isolation.
+	require.NoError(t, m.Steps(-1), "down 014 drops channel_title, login_events intact")
+	require.True(t, loginEventsExists(t), "014 down leaves login_events intact")
 	require.NoError(t, m.Steps(-1), "down 013 drops channel_errors, login_events intact")
 	require.True(t, loginEventsExists(t), "013 down leaves login_events intact")
 	require.NoError(t, m.Steps(-1), "down 012 is a no-op, login_events intact")
@@ -64,7 +66,7 @@ func TestMigration010LoginEventsUpDown(t *testing.T) {
 	require.NoError(t, m.Steps(-1), "down 010 must drop login_events")
 	require.False(t, loginEventsExists(t), "login_events must be gone after the down migration")
 
-	require.NoError(t, m.Steps(4), "up must recreate 010 then re-apply 011, 012, 013")
+	require.NoError(t, m.Steps(5), "up must recreate 010 then re-apply 011, 012, 013, 014")
 	require.True(t, loginEventsExists(t), "login_events must be restored after the up migration")
 }
 
@@ -87,6 +89,7 @@ func TestMigration011AutoSummarizeDefaultUpDown(t *testing.T) {
 	require.Equal(t, "true", autoSummarizeDefault(t), "011 sets the default to TRUE")
 
 	m := fileMigrator(t)
+	require.NoError(t, m.Steps(-1), "down 014 drops channel_title")
 	require.NoError(t, m.Steps(-1), "down 013 drops channel_errors")
 	require.NoError(t, m.Steps(-1), "down 012 is a no-op")
 	require.NoError(t, m.Steps(-1), "down 011 reverts the column default")
@@ -96,6 +99,31 @@ func TestMigration011AutoSummarizeDefaultUpDown(t *testing.T) {
 	require.Equal(t, "true", autoSummarizeDefault(t))
 	require.NoError(t, m.Steps(1), "up 012 runs clean (no FORCE RLS on fresh schema)")
 	require.NoError(t, m.Steps(1), "up 013 creates channel_errors")
+	require.NoError(t, m.Steps(1), "up 014 recreates channel_title")
+}
+
+// channelTitleExists reports whether videos.channel_title is present.
+func channelTitleExists(t *testing.T) bool {
+	t.Helper()
+	var exists bool
+	require.NoError(t, rawPool(t).QueryRow(context.Background(),
+		`SELECT EXISTS (SELECT 1 FROM information_schema.columns
+		  WHERE table_name = 'videos' AND column_name = 'channel_title')`).Scan(&exists))
+	return exists
+}
+
+// TestMigration014VideoChannelTitleUpDown proves 014 is reversible: down drops
+// videos.channel_title, up recreates it.
+func TestMigration014VideoChannelTitleUpDown(t *testing.T) {
+	newStore(t) // latest (014 applied)
+	require.True(t, channelTitleExists(t), "channel_title exists at latest migration")
+
+	m := fileMigrator(t)
+	require.NoError(t, m.Steps(-1), "down 014 must drop channel_title")
+	require.False(t, channelTitleExists(t), "channel_title must be gone after the down migration")
+
+	require.NoError(t, m.Steps(1), "up 014 must recreate channel_title")
+	require.True(t, channelTitleExists(t), "channel_title must be restored after the up migration")
 }
 
 // TestMigration012FixAutoSummarizeRLS proves 012 runs cleanly and flips any