feat(transcribe): Ensemble -- two local engines, a local reconciler, fail-soft at every step

Runs the fast engine then the other from one WAV and reconciles; one engine down
uses the other and says so; the model down gives the base text unreconciled; the
cloud fallback runs only when BOTH local engines fail and is recorded as such; a
cancelled job stops everything and never reaches the cloud. The endpoint lists every
contributor. Mutation-checked, including both cancellation points.

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

internal/adapters/transcribe/ensemble.go

@@ -0,0 +1,77 @@
+package transcribe
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	"git.d-ma.be/mathias/tapir/internal/domain"
+	"git.d-ma.be/mathias/tapir/internal/reconcile"
+)
+
+// Ensemble transcribes the same WAV with two engines, one after the other, and has a
+// local model reconcile their disagreements (ADR-034). It satisfies uploads.Transcriber,
+// so the worker neither knows nor cares that two engines were involved.
+//
+// It degrades in steps and never turns a recoverable problem into a failed job:
+// one engine down → the other's text, said so on the result; the model down → the
+// base text unreconciled; both engines down → the cloud fallback, if configured, and
+// recorded as such. A cancelled job stops everything and never reaches the cloud.
+type Ensemble struct {
+	base, alt Backend
+	fallback  Backend // nil = no cloud fallback
+	adj       reconcile.Adjudicator
+	opts      reconcile.Options
+}
+
+// NewEnsemble builds an Ensemble. base is the engine whose text stands where the two
+// disagree and is the faster one, so it runs first; alt supplies the other reading.
+func NewEnsemble(base, alt, fallback Backend, adj reconcile.Adjudicator, opts reconcile.Options) *Ensemble {
+	return &Ensemble{base: base, alt: alt, fallback: fallback, adj: adj, opts: opts}
+}
+
+// Transcribe implements uploads.Transcriber.
+func (e *Ensemble) Transcribe(ctx context.Context, wavPath string) (domain.Transcription, error) {
+	b, berr := e.base.Transcribe(ctx, wavPath)
+	if err := ctx.Err(); err != nil {
+		return domain.Transcription{}, err
+	}
+	a, aerr := e.alt.Transcribe(ctx, wavPath)
+	if err := ctx.Err(); err != nil {
+		return domain.Transcription{}, err
+	}
+
+	switch {
+	case berr == nil && aerr == nil:
+		out, err := reconcile.Reconcile(ctx, b, a, e.adj, e.opts)
+		if err != nil {
+			return domain.Transcription{}, err
+		}
+		out.Endpoint = fmt.Sprintf("%s + %s, reconciled by %s", e.base.Name(), e.alt.Name(), e.opts.Reconciler)
+		return out, nil
+	case berr == nil:
+		return e.single(b, e.base.Name(), fmt.Sprintf("%s failed: %v", e.alt.Name(), aerr)), nil
+	case aerr == nil:
+		return e.single(a, e.alt.Name(), fmt.Sprintf("%s failed: %v", e.base.Name(), berr)), nil
+	}
+
+	both := errors.Join(fmt.Errorf("%s: %w", e.base.Name(), berr), fmt.Errorf("%s: %w", e.alt.Name(), aerr))
+	if e.fallback == nil {
+		return domain.Transcription{}, both
+	}
+	f, ferr := e.fallback.Transcribe(ctx, wavPath)
+	if ferr != nil {
+		return domain.Transcription{}, errors.Join(both, fmt.Errorf("%s: %w", e.fallback.Name(), ferr))
+	}
+	return e.single(f, e.fallback.Name(), "both local engines failed"), nil
+}
+
+// single is a transcript from one engine only: nothing to reconcile, and the result
+// says why, because a reader should know the second opinion was missing.
+func (e *Ensemble) single(t domain.Transcription, engine, reason string) domain.Transcription {
+	t.Endpoint = engine
+	t.Reconciliation = &domain.Reconciliation{
+		Status: domain.ReconcileSingle, Reason: reason, Base: e.opts.BaseEngine, Alt: e.opts.AltEngine, Reconciler: e.opts.Reconciler,
+	}
+	return t
+}