feat(onboard): burst picks likely-good videos, summarizes with stronger model (ADR-028)

Wire the onboarding burst to its quality-aware selection and a stronger model:
- main.go onboard uses OnboardBurstVideoIDs (junk-avoiding) instead of pure
  newest-first, bounded by MinVideoSeconds / OnboardMaxVideoSeconds.
- buildBurstProcessor builds a burst-only summarizer chain led by the onboard
  model (burstChainModels: onboard -> standard ADR-022 chain, deduped, NDA lever
  intact), over the same store/cache/sink. Collapses onto the shared Processor
  when the onboard model is empty/equal-to-primary or config is incomplete.
- summarizerEndpoint + newYouTubeSource extracted so standard and burst wiring
  share one definition.
- Remove now-superseded NewestUnsummarizedVideoIDs: OnboardBurstVideoIDs(.,0,0)
  is identical pure-newest behaviour and its test covers RLS + ordering.

internal/adapters/store/videos_test.go

@@ -112,38 +112,6 @@ func TestUpsertVideo_PerUserIsolation(t *testing.T) {
 	require.NotEqual(t, idA, idB, "same provider video for two users must be two distinct rows")
 }
 
-func TestNewestUnsummarizedVideoIDs(t *testing.T) {
-	ctx := context.Background()
-	s := newStore(t)
-	resetDB(t, rawPool(t))
-
-	mk := func(user, pid string, day int) string {
-		v := ytVideo(user, pid, pid)
-		v.PublishedAt = time.Date(2026, 6, day, 12, 0, 0, 0, time.UTC)
-		id, err := s.UpsertVideo(ctx, v)
-		require.NoError(t, err)
-		return id
-	}
-
-	_ = mk(userA, "a1vid000001", 1)
-	id2 := mk(userA, "a2vid000002", 2)
-	id3 := mk(userA, "a3vid000003", 3)
-	id4 := mk(userA, "a4vid000004", 4)
-	mk(userB, "b1vid000009", 9) // userB's newest — must never leak via RLS
-
-	// The newest (v4) is summarized, so it's excluded from "unsummarized".
-	require.NoError(t, s.Deliver(ctx, summary(userA, id4, "done")))
-
-	// Cap 2, newest-first unsummarized: v3 then v2 (v4 excluded; userB excluded).
-	got, err := s.NewestUnsummarizedVideoIDs(ctx, userA, 2)
-	require.NoError(t, err)
-	require.Equal(t, []string{id3, id2}, got)
-
-	none, err := s.NewestUnsummarizedVideoIDs(ctx, userA, 0)
-	require.NoError(t, err)
-	require.Empty(t, none, "limit 0 returns nothing")
-}
-
 func TestOnboardBurstVideoIDs(t *testing.T) {
 	ctx := context.Background()
 	s := newStore(t)
@@ -158,14 +126,16 @@ func TestOnboardBurstVideoIDs(t *testing.T) {
 		return id
 	}
 
-	good1 := mk(userA, "good0000001", 5, 600)    // 10m, newest known-good
-	tooLong := mk(userA, "toolong0001", 4, 20000) // > maxSeconds -> dropped
-	_ = tooLong
-	tooShort := mk(userA, "tooshort001", 3, 30) // < minSeconds -> dropped
-	_ = tooShort
-	unknown := mk(userA, "unknown0001", 2, 0)  // NULL duration -> kept, ranked last
-	good2 := mk(userA, "good0000002", 1, 800)  // known-good but oldest
-	mk(userB, "bvid0000009", 9, 600)           // userB -> must not leak via RLS
+	summarized := mk(userA, "summ0000001", 6, 600) // newest known-good, but already summarized
+	good1 := mk(userA, "good0000001", 5, 600)      // 10m, newest UNsummarized known-good
+	tooLong := mk(userA, "toolong0001", 4, 20000)  // > maxSeconds -> dropped
+	tooShort := mk(userA, "tooshort001", 3, 30)    // < minSeconds -> dropped
+	unknown := mk(userA, "unknown0001", 2, 0)      // NULL duration -> kept, ranked last
+	good2 := mk(userA, "good0000002", 1, 800)      // known-good but oldest
+	mk(userB, "bvid0000009", 9, 600)               // userB -> must not leak via RLS
+
+	// The newest video is summarized, so it is excluded from the burst.
+	require.NoError(t, s.Deliver(ctx, summary(userA, summarized, "done")))
 
 	const minSec, maxSec = 60, 14400