feat(transcribe): WithTimeout -- each engine call gets its own deadline so a wedged engine fails while the job lives (review F4)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/adapters/transcribe/timeout.go

@@ -0,0 +1,33 @@
+package transcribe
+
+import (
+	"context"
+	"time"
+
+	"git.d-ma.be/mathias/tapir/internal/domain"
+)
+
+// WithTimeout gives every call to b its own deadline. A wedged engine does not answer at
+// all (iguana after a Metal OOM, infra#273), and with only the job's deadline it used up
+// the whole job before a fallback could run -- and Chain then refuses the fallback because
+// the job context has expired. With its own deadline the call fails while the job is still
+// alive, which is exactly when the fallback, or the other engine, can take over.
+//
+// A zero or negative d returns b unchanged. The wrapper is invisible: Name is b's.
+func WithTimeout(b Backend, d time.Duration) Backend {
+	if d <= 0 {
+		return b
+	}
+	return timeoutBackend{Backend: b, d: d}
+}
+
+type timeoutBackend struct {
+	Backend
+	d time.Duration
+}
+
+func (t timeoutBackend) Transcribe(ctx context.Context, wavPath string) (domain.Transcription, error) {
+	ctx, cancel := context.WithTimeout(ctx, t.d)
+	defer cancel()
+	return t.Backend.Transcribe(ctx, wavPath)
+}