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/web/handlers_test.go

@@ -207,13 +207,15 @@ func TestListChannelFilter(t *testing.T) {
 	resetDB(t, p)
 	require.NoError(t, deliver(ctx, app, videoX, "body x"))
 	seedVideo(t, p, videoX, "X Title", "https://x", time.Time{})
+	_, err := p.Exec(ctx, `UPDATE videos SET channel_title = 'Acme Channel' WHERE id = $1`, videoX)
+	require.NoError(t, err)
 
-	// Channel is "youtube" for seeded rows; a non-matching filter hides them.
-	rec := do(t, app, httptest.NewRequest(http.MethodGet, "/?channel=vimeo", nil))
+	// Selecting a different channel hides the row; selecting its channel shows it.
+	rec := do(t, app, httptest.NewRequest(http.MethodGet, "/?channel=Other+Channel", nil))
 	require.Equal(t, http.StatusOK, rec.Code)
 	require.NotContains(t, body(t, rec), "X Title")
 
-	rec = do(t, app, httptest.NewRequest(http.MethodGet, "/?channel=youtube", nil))
+	rec = do(t, app, httptest.NewRequest(http.MethodGet, "/?channel=Acme+Channel", nil))
 	require.Contains(t, body(t, rec), "X Title")
 }