fix(security): close confirmed findings from the 2026-10-15 hunt (#166)

Seven findings are fixed. Each fix has a regression test.

- Compare X-Forwarded-Host and the serving host case-insensitively, so a
  case variant cannot widen the session cookie Domain.
- Reject a future iat on the opaque introspection path.
- Serve the cached parsed JWKS inside the signature-refresh cooldown.
- Release the half-open circuit slot when a probe panics.
- Cap requests that wait for provider initialisation at 512. Extra
  requests receive 503 with Retry-After.
- Cap cookie deletion headers at 32 cookie names per response.
- Cap front-channel sid markers at 1000, so unauthenticated requests
  cannot evict backchannel or jti replay markers.

internal/recovery/circuit_breaker.go

@@ -93,8 +93,16 @@ func (cb *CircuitBreaker) ExecuteWithContext(ctx context.Context, fn func() erro
 		return fmt.Errorf("circuit breaker is open")
 	}
 
-	// Execute the function
+	// A panicking fn must still settle the breaker, otherwise a half-open
+	// probe slot leaks until the next open->half-open transition.
+	panicked := true
+	defer func() {
+		if panicked {
+			cb.recordFailure()
+		}
+	}()
 	err := fn()
+	panicked = false
 
 	if err != nil {
 		cb.recordFailure()

internal/recovery/hunt_20261015_regression_test.go

@@ -0,0 +1,40 @@
+package recovery
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+)
+
+func TestCircuitBreaker_PanickingProbeReleasesHalfOpenSlot(t *testing.T) {
+	cb := NewCircuitBreaker(CircuitBreakerConfig{
+		FailureThreshold: 1,
+		SuccessThreshold: 1,
+		Timeout:          20 * time.Millisecond,
+		MaxRequests:      1,
+	}, &mockLogger{})
+
+	_ = cb.ExecuteWithContext(context.Background(), func() error { return errors.New("fail") })
+	time.Sleep(40 * time.Millisecond)
+
+	func() {
+		defer func() {
+			if recover() == nil {
+				t.Error("panic must still propagate to the caller")
+			}
+		}()
+		_ = cb.ExecuteWithContext(context.Background(), func() error { panic("boom") })
+	}()
+
+	if cb.GetState() != CircuitBreakerOpen {
+		t.Fatalf("state after panicking probe = %s, want open", cb.GetState())
+	}
+	time.Sleep(40 * time.Millisecond)
+	if err := cb.ExecuteWithContext(context.Background(), func() error { return nil }); err != nil {
+		t.Fatalf("probe after next timeout rejected: %v", err)
+	}
+	if cb.GetState() != CircuitBreakerClosed {
+		t.Fatalf("state = %s, want closed", cb.GetState())
+	}
+}