feat(web): paste-a-URL handler — add an arbitrary video + summarize (Feature 2)

POST /paste: parse the video id, fetch metadata via the VideoFetcher port (Data
API, ungated), upsert a subscription-less row scoped to the user (idempotent =
dedup), and — unless already summarized — RequestSummarize + start immediate
processing through the SAME globalFetchGate as the Summarize button. Explicit
paste overrides the recency window; a captionless video degrades to the honest
'no transcript' terminal state via the engine (ADR-010). Invalid URL -> 400,
not-found -> 404, both add nothing. Route mounts only when a Fetcher is wired.

Moves the video-not-found sentinel to domain (shared by adapter + web, no
cross-adapter coupling). Tests: valid add+queue, invalid, not-found, dedup.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/adapters/youtube/videobyid_test.go

@@ -53,7 +53,7 @@ func TestVideoByIDNotFound(t *testing.T) {
 		_, _ = w.Write([]byte(`{"items":[]}`))
 	})
 	_, err := a.VideoByID(context.Background(), "u1", "missingvid0")
-	if !errors.Is(err, ErrVideoNotFound) {
-		t.Fatalf("VideoByID for missing id = %v, want ErrVideoNotFound", err)
+	if !errors.Is(err, domain.ErrVideoNotFound) {
+		t.Fatalf("VideoByID for missing id = %v, want domain.ErrVideoNotFound", err)
 	}
 }