feat: add use-case engine scaffold (RED)

Engine wires the ports and exposes ProcessNewVideo, the core use case. Returns
ErrNotImplemented on purpose so the acceptance suite fails RED — implementing it
to make those tests pass is the first build task. Depends only on ports + domain
(dependencies point inward).

internal/usecase/engine.go

@@ -0,0 +1,47 @@
+// Package usecase holds Tapir's application core: the engine that detects new
+// videos, resolves transcripts, summarizes, and delivers to sinks. It depends
+// only on internal/ports and internal/domain.
+//
+// This is a SCAFFOLD. Methods return ErrNotImplemented so the acceptance tests
+// in test/acceptance fail RED. Implementing them to make those tests pass is
+// the first build task (see the build issue). Behaviour is specified in
+// docs/use-cases/*.feature.
+package usecase
+
+import (
+	"context"
+	"errors"
+
+	"gitea.d-ma.be/mathias/tapir/internal/domain"
+	"gitea.d-ma.be/mathias/tapir/internal/ports"
+)
+
+// ErrNotImplemented marks scaffold methods awaiting implementation.
+var ErrNotImplemented = errors.New("not implemented")
+
+// Engine is the provider- and sink-agnostic core.
+type Engine struct {
+	Source ports.VideoSource
+	AI     ports.Summarizer
+	Sinks  []ports.Sink
+}
+
+// NewEngine wires the engine from its ports.
+func NewEngine(src ports.VideoSource, ai ports.Summarizer, sinks ...ports.Sink) *Engine {
+	return &Engine{Source: src, AI: ai, Sinks: sinks}
+}
+
+// ProcessResult reports what happened for one video.
+type ProcessResult struct {
+	Video   domain.Video
+	Skipped bool
+	Reason  string          // set when Skipped (e.g. "no transcript")
+	Summary *domain.Summary // nil when Skipped
+}
+
+// ProcessNewVideo runs the core use case for a single video:
+// resolve transcript -> (summarize -> deliver) | skip.
+// See docs/use-cases/summarize_new_video.feature.
+func (e *Engine) ProcessNewVideo(ctx context.Context, v domain.Video) (ProcessResult, error) {
+	return ProcessResult{}, ErrNotImplemented
+}