fix(scheduler): rotate over connected users only for true fair share

The lead-user rotation rotated the full ListAllUsers set, so a connectionless
orphan identity ate a rotation slot — collapsing onto the next real user and
skewing the lead share (two real users got 2/3 vs 1/3 instead of 50/50). Filter
to connected users BEFORE rotating so the rotation is over exactly the users that
consume the caption budget. A dead identity can no longer skew fairness.

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

cmd/tapir/scheduler.go

@@ -73,22 +73,17 @@ 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
+	// Keep only users with a video connection. A pass for a connectionless user
+	// (e.g. a stale Dex-era orphan identity) only tries to resolve a token that
+	// was never minted, logging a spurious "ref not found" every tick. Filtering
+	// here — BEFORE rotation — also keeps fairness honest: rotation is over the
+	// users that actually consume the caption budget, so a dead identity can't eat
+	// a rotation slot and skew the lead share.
+	var connected []store.UserIdentity
 	for _, u := range users {
 		if ctx.Err() != nil {
-			break // shutting down: stop enumerating
+			return runner.Stats{} // shutting down
 		}
-		// Skip users with no video connection. A discovery pass for them only
-		// attempts to resolve a token that was never minted, logging a spurious
-		// "ref not found" every tick (e.g. stale Dex-era orphan identities).
 		conns, err := lister.ConnectionsForUser(ctx, u.UserID)
 		if err != nil {
 			log.Warn("scheduler: list connections failed", "user", u.UserID, "err", err)
@@ -98,6 +93,22 @@ func runDiscoveryPass(
 			log.Debug("scheduler: skipping user with no video connections", "user", u.UserID)
 			continue
 		}
+		connected = append(connected, u)
+	}
+
+	// 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 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 over the connected set gives each real user the lead in turn.
+	connected = rotateUsers(connected, pass)
+
+	log.Info("scheduler: starting discovery pass", "users", len(connected))
+	var total runner.Stats
+	for _, u := range connected {
+		if ctx.Err() != nil {
+			break // shutting down: stop enumerating
+		}
 		stats, err := runUser(ctx, u.UserID)
 		total = sumStats(total, stats)
 		if err != nil {