fix(scheduler): cache transcripts on the scheduled path (ADR-021 regression)

buildUserRunner built the engine without engine.Transcripts = st, so the
scheduler — unlike the web "Summarize now" path — never read or wrote the shared
transcript cache. Every discovery pass re-fetched transcripts it had already
fetched, burning the scarce per-egress-IP caption budget (ADR-014) on redundant
work and starving other users' first-time fetches. The transcripts table was
empty despite summaries existing. Wire the cache on this path too.

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

cmd/tapir/scheduler.go

@@ -36,6 +36,11 @@ func buildUserRunner(cfg config.Config, st *store.Store, secretStore ports.Secre
 	}, secretStore)
 
 	engine := usecase.NewEngine(src, buildSummarizer(cfg), st)
+	// Share the transcript cache (ADR-021) on the scheduler path too — without
+	// this every scheduled pass re-fetches transcripts it already had, burning the
+	// scarce per-IP caption budget (ADR-014) and starving other users. The web
+	// "Summarize now" path already sets this; the scheduler omitting it was a bug.
+	engine.Transcripts = st
 
 	return runner.New(src, st, engine, userID, log,
 		runner.WithBackoff(cfg.FetchBackoff),
@@ -57,6 +62,7 @@ type userLister interface {
 // isolation). Returns the stats summed across users.
 func runDiscoveryPass(
 	ctx context.Context,
+	pass int,
 	lister userLister,
 	runUser func(context.Context, string) (runner.Stats, error),
 	log *slog.Logger,
@@ -67,6 +73,13 @@ func runDiscoveryPass(
 		return runner.Stats{}
 	}
 
+	// Rotate who goes first each pass. Caption fetches share one per-egress-IP
+	// rate budget (ADR-014); whoever runs first each pass spends the pre-throttle
+	// window, so a FIXED user order permanently starves whoever is last (a new
+	// pilot user got 0 fetches for 12h while the first-listed user got all of
+	// them). Rotation gives every user the lead slot in turn.
+	users = rotateUsers(users, pass)
+
 	log.Info("scheduler: starting discovery pass", "users", len(users))
 	var total runner.Stats
 	for _, u := range users {
@@ -116,7 +129,8 @@ func runScheduler(
 		return // disabled
 	}
 
-	runDiscoveryPass(ctx, lister, runUser, log)
+	pass := 0
+	runDiscoveryPass(ctx, pass, lister, runUser, log)
 
 	ticker := time.NewTicker(interval)
 	defer ticker.Stop()
@@ -125,11 +139,32 @@ func runScheduler(
 		case <-ctx.Done():
 			return
 		case <-ticker.C:
-			runDiscoveryPass(ctx, lister, runUser, log)
+			pass++
+			runDiscoveryPass(ctx, pass, lister, runUser, log)
 		}
 	}
 }
 
+// rotateUsers left-rotates users by pass positions so a different user leads each
+// pass. With n users, user i leads on every pass where pass ≡ i (mod n). A pass
+// offset that is negative or exceeds n is normalised. Order within the rotation
+// is otherwise preserved, so the set of users run is unchanged — only who is
+// first (and thus wins the scarce caption-fetch budget) rotates.
+func rotateUsers(users []store.UserIdentity, pass int) []store.UserIdentity {
+	n := len(users)
+	if n <= 1 {
+		return users
+	}
+	off := ((pass % n) + n) % n
+	if off == 0 {
+		return users
+	}
+	out := make([]store.UserIdentity, 0, n)
+	out = append(out, users[off:]...)
+	out = append(out, users[:off]...)
+	return out
+}
+
 // sumStats adds two passes' stats field-wise, so runDiscoveryPass can report a
 // per-tick aggregate across all users.
 func sumStats(a, b runner.Stats) runner.Stats {