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.go

@@ -21,6 +21,9 @@ import (
 // fake without a database.
 type Store interface {
 	ListVideos(ctx context.Context, userID string, limit int) ([]store.SummaryRow, error)
+	// DistinctChannels lists the user's source channels — the options for the
+	// feed's channel multi-select filter.
+	DistinctChannels(ctx context.Context, userID string) ([]string, error)
 	GetSummaryByVideo(ctx context.Context, userID, videoID string) (*store.SummaryRow, error)
 	GetVideoRow(ctx context.Context, userID, videoID string) (*store.SummaryRow, error)
 	ActionsFor(ctx context.Context, userID string, videoIDs []string) (map[string][]string, error)
@@ -196,7 +199,7 @@ func (a *App) handleList(w http.ResponseWriter, r *http.Request) {
 	}
 	q := r.URL.Query()
 	f := Filter{
-		Channel:        q.Get("channel"),
+		Channels:       nonEmptyStrings(q["channel"]),
 		From:           q.Get("from"),
 		To:             q.Get("to"),
 		OnlySummarized: q.Get("summarized") == "1",
@@ -211,6 +214,13 @@ func (a *App) handleList(w http.ResponseWriter, r *http.Request) {
 	rows := f.apply(allRows)
 	buckets := bucketRows(rows, a.recencyCutoff())
 
+	// Channel options for the multi-select filter (the user's source channels).
+	channels, err := a.Store.DistinctChannels(r.Context(), userID)
+	if err != nil {
+		a.serverError(w, r, "distinct channels", err)
+		return
+	}
+
 	// hasConnected drives both the paste box (shown to ANY connected user, #2) and
 	// the empty-state copy (a fresh account with a connection but no discovery pass
 	// yet reads "connected, summaries land gradually" rather than "nothing here").
@@ -227,7 +237,7 @@ func (a *App) handleList(w http.ResponseWriter, r *http.Request) {
 		a.render(w, r, summaryList(buckets, hasConnected))
 		return
 	}
-	a.render(w, r, ListPage(buckets, f, stats, takeFlash(w, r), hasConnected))
+	a.render(w, r, ListPage(buckets, f, stats, takeFlash(w, r), hasConnected, channels))
 }
 
 // handleDetail renders one summary in full (highlights, takeaways, action group).