feat(gate): foreground caption fetches take priority over the background sweep (ADR-026)

A user waiting on a Summarize click shared the per-IP caption gate equally with
the background firehose, so on a busy IP the click was slow or 429'd. Add a
context-marked priority lane: the web path (engineProcessor.ProcessVideo) marks
its context foreground; the gate serves foreground immediately while background
fetches yield until no foreground is pending. Threaded via a context value (no
new signatures) + a process-wide foregroundPending counter. Clicks are rare, so
the background barely loses throughput; the waiting human gets the cleaner slot.

Drops the credentials probe: ADR-010 and captions.go already settle it — the
timedtext/InnerTube path rejects authenticated requests and the OAuth token does
not authenticate it anyway, so auth cannot help and can hurt. Documented in
ADR-026 rather than built.

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

internal/adapters/youtube/gate.go

@@ -2,6 +2,7 @@ package youtube
 
 import (
 	"context"
+	"sync/atomic"
 	"time"
 
 	"golang.org/x/time/rate"
@@ -29,9 +30,55 @@ func SetFetchRate(interval time.Duration) {
 	globalFetchGate = rate.NewLimiter(rate.Every(interval), 1)
 }
 
+// foregroundPending counts in-flight foreground (user-initiated) caption fetches.
+// The background sweep yields the gate while this is non-zero so a human waiting
+// on a click gets the next slot — and, on a near-throttled IP, the pre-429 window
+// — instead of competing equally with the firehose (ADR-026, Pillar A). Clicks are
+// rare and bursty, so background barely notices; the win to the click is large.
+var foregroundPending atomic.Int64
+
+// fgCtxKey marks a context as foreground (user-initiated). Unexported; set via
+// ForegroundContext and read via isForeground so only this package owns the key.
+type fgCtxKey struct{}
+
+// ForegroundContext marks ctx as a user-initiated (foreground) fetch so the gate
+// gives it priority. The web "Summarize"/paste/retry path wraps its context with
+// this; the background scheduler leaves it unset.
+func ForegroundContext(ctx context.Context) context.Context {
+	return context.WithValue(ctx, fgCtxKey{}, true)
+}
+
+func isForeground(ctx context.Context) bool {
+	v, _ := ctx.Value(fgCtxKey{}).(bool)
+	return v
+}
+
+// fgYieldPoll is how often a background waiter re-checks whether a foreground
+// fetch is still pending. Short enough to feel immediate, long enough not to spin.
+const fgYieldPoll = 200 * time.Millisecond
+
 // WaitFetchGate blocks until the process-wide gate allows one timedtext fetch,
 // respecting ctx cancellation. Called from httpDo before every live outbound
 // caption fetch so the scheduler and the click-path share the same egress budget.
+//
+// Foreground (user-initiated) fetches take priority: they register as pending and
+// acquire a token immediately. Background fetches first yield — they wait until no
+// foreground fetch is pending — so a live click is never stuck behind the
+// background sweep and gets the cleaner slot against the per-IP limit (ADR-026).
 func WaitFetchGate(ctx context.Context) error {
+	if isForeground(ctx) {
+		foregroundPending.Add(1)
+		defer foregroundPending.Add(-1)
+		return globalFetchGate.Wait(ctx)
+	}
+
+	// Background: defer to any pending foreground fetch before taking a token.
+	for foregroundPending.Load() > 0 {
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		case <-time.After(fgYieldPoll):
+		}
+	}
 	return globalFetchGate.Wait(ctx)
 }