feat(web): pipeline stats bar, summarized-first sort, Try now button for rate-limited videos

Three UX improvements for the pending-transcript state:
1. Summarized videos sort to top (ORDER BY (s.id IS NOT NULL) DESC, seen_at DESC)
   so completed summaries are always immediately visible without filtering.
   ListVideos default limit raised from 50 to 500 to show the full backlog.
2. Pipeline stats bar above the video list: '2 summarized · 256 fetching soon · 12
   no captions' — computed from the unfiltered row set, hidden when everything is
   summarized.
3. 'Try now' button on rate-limited cards replaces the passive 'Retrying later'
   chip. POST /v/{id}/retry-now clears rate_limited_at then calls ProcessVideo
   through the shared globalFetchGate — same rate limiting as the scheduler, safe
   under concurrent use.

internal/web/videocard_internal_test.go

@@ -17,8 +17,8 @@ func renderVideoCard(t *testing.T, r store.SummaryRow) string {
 	return sb.String()
 }
 
-// A rate-limited, unsummarized video shows the passive "Retrying later" badge and
-// hides the Summarize button — the user can't fix it, retry is automatic.
+// A rate-limited, unsummarized video shows an active "Try now" button so the user
+// can manually trigger an immediate fetch through the shared rate gate.
 func TestVideoCard_RateLimitedShowsRetryingBadge(t *testing.T) {
 	html := renderVideoCard(t, store.SummaryRow{
 		VideoID:          "aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa",
@@ -27,11 +27,11 @@ func TestVideoCard_RateLimitedShowsRetryingBadge(t *testing.T) {
 		TranscriptStatus: "rate_limited",
 	})
 
-	if !strings.Contains(html, "Retrying later") {
-		t.Errorf("expected a 'Retrying later' badge, got:\n%s", html)
+	if !strings.Contains(html, "Try now") {
+		t.Errorf("expected a 'Try now' button, got:\n%s", html)
 	}
-	if !strings.Contains(html, "chip-retry") {
-		t.Errorf("expected the passive chip-retry styling, got:\n%s", html)
+	if !strings.Contains(html, "retry-now") {
+		t.Errorf("expected the retry-now route in the form action, got:\n%s", html)
 	}
 	if strings.Contains(html, ">Summarize<") {
 		t.Errorf("the Summarize button must be hidden for a rate-limited video, got:\n%s", html)