feat(seeding): wire seed ratio/time lifecycle into the torrent daemon

SeedRatio/SeedTime were declared on TorrentConfig but never consumed, and
SeedEnabled was hardcoded false in both constructors — the daemon never
seeded, and if forced it seeded forever.

- config: [downloads] seed_enabled/seed_ratio/seed_time (opt-in, off by default)
- daemon: parse seed_time + wire all three; startup log per target shape
- engine: seedTargetReached() (pure) + seedAndDrop() background monitor on a
  downloader-scoped seedCtx (not the task ctx, which dies when Download returns);
  drops the torrent on ratio (uploaded/size) OR time, whichever first; no target
  = seed until shutdown. Configurable check interval (tests lower it).
- fix: cleanup() now always drops — previously leaked the handle on error paths
  when seeding was enabled.
- refactor: dropTracked() helper shared by cleanup + post-seeding drop.

Tests: TestSeedTargetReached (9 cases) + ctx/no-target branches + loopback
swarm smoke (-tags smoke). Roadmap hueco closed.

internal/engine/torrent_test.go

@@ -264,3 +264,114 @@ func TestTorrentDownloader_DownloadTimeout_MetadataCancel(t *testing.T) {
 func TestTorrentDownloader_ImplementsInterface(t *testing.T) {
 	var _ Downloader = (*TorrentDownloader)(nil)
 }
+
+// TestSeedTargetReached cubre la lógica pura de parada del seeding: ratio,
+// tiempo, ninguno, ambos (el primero que se cumple gana) y la guarda de tamaño
+// cero (no debe dividir por cero ni parar por ratio).
+func TestSeedTargetReached(t *testing.T) {
+	tests := []struct {
+		name        string
+		ratioTarget float64
+		timeTarget  time.Duration
+		uploaded    int64
+		size        int64
+		elapsed     time.Duration
+		wantStop    bool
+	}{
+		{"ratio reached", 2.0, 0, 200, 100, time.Minute, true},
+		{"ratio not reached", 2.0, 0, 150, 100, time.Minute, false},
+		{"ratio exactly met", 1.0, 0, 100, 100, time.Minute, true},
+		{"time reached", 0, time.Hour, 10, 100, 2 * time.Hour, true},
+		{"time not reached", 0, time.Hour, 10, 100, 30 * time.Minute, false},
+		{"no targets never stops", 0, 0, 9999, 100, 99 * time.Hour, false},
+		{"ratio wins when both set", 2.0, time.Hour, 200, 100, time.Second, true},
+		{"time wins when ratio short", 5.0, time.Hour, 100, 100, 2 * time.Hour, true},
+		{"zero size guards div", 2.0, 0, 200, 0, time.Minute, false},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			reason := seedTargetReached(tc.ratioTarget, tc.timeTarget, tc.uploaded, tc.size, tc.elapsed)
+			if got := reason != ""; got != tc.wantStop {
+				t.Errorf("seedTargetReached(ratio=%.1f time=%s up=%d size=%d el=%s) stop=%v (reason %q), want %v",
+					tc.ratioTarget, tc.timeTarget, tc.uploaded, tc.size, tc.elapsed, got, reason, tc.wantStop)
+			}
+		})
+	}
+}
+
+// TestTorrentDownloader_SeedRatioTime verifica que SeedRatio y SeedTime se
+// propagan a la config del downloader.
+func TestTorrentDownloader_SeedRatioTime(t *testing.T) {
+	dir := t.TempDir()
+	dl, err := NewTorrentDownloader(TorrentConfig{
+		DataDir:     dir,
+		SeedEnabled: true,
+		SeedRatio:   1.5,
+		SeedTime:    2 * time.Hour,
+	})
+	if err != nil {
+		t.Fatalf("NewTorrentDownloader: %v", err)
+	}
+	defer dl.Shutdown(context.Background())
+
+	if dl.cfg.SeedRatio != 1.5 {
+		t.Errorf("SeedRatio = %v, want 1.5", dl.cfg.SeedRatio)
+	}
+	if dl.cfg.SeedTime != 2*time.Hour {
+		t.Errorf("SeedTime = %v, want 2h", dl.cfg.SeedTime)
+	}
+	if dl.seedCtx == nil || dl.seedCancel == nil {
+		t.Error("seedCtx/seedCancel must be initialised by the constructor")
+	}
+}
+
+// TestSeedAndDrop_NoTargetReturnsImmediately verifica que sin ratio ni tiempo
+// objetivo, seedAndDrop retorna de inmediato (siembra indefinida) sin tocar el
+// handle — por eso es seguro pasar un torrent nil.
+func TestSeedAndDrop_NoTargetReturnsImmediately(t *testing.T) {
+	dir := t.TempDir()
+	dl, err := NewTorrentDownloader(TorrentConfig{DataDir: dir, SeedEnabled: true}) // ratio 0, time 0
+	if err != nil {
+		t.Fatalf("NewTorrentDownloader: %v", err)
+	}
+	defer dl.Shutdown(context.Background())
+
+	done := make(chan struct{})
+	go func() {
+		dl.seedAndDrop("no-target-task-id", nil, 1000)
+		close(done)
+	}()
+
+	select {
+	case <-done:
+	case <-time.After(2 * time.Second):
+		t.Fatal("seedAndDrop with no target should return immediately")
+	}
+}
+
+// TestSeedAndDrop_StopsOnSeedCtxCancel verifica que seedAndDrop sale cuando se
+// cancela seedCtx (ruta de Shutdown), incluso con un objetivo de ratio alto y el
+// tick deshabilitado — el único camino de salida es seedCtx.Done().
+func TestSeedAndDrop_StopsOnSeedCtxCancel(t *testing.T) {
+	dir := t.TempDir()
+	dl, err := NewTorrentDownloader(TorrentConfig{DataDir: dir, SeedEnabled: true, SeedRatio: 99})
+	if err != nil {
+		t.Fatalf("NewTorrentDownloader: %v", err)
+	}
+	defer dl.Shutdown(context.Background())
+
+	dl.seedCheckInterval = time.Hour // el ticker no disparará; solo seedCtx.Done() puede terminar
+	dl.seedCancel()                  // cancela antes de arrancar el monitor
+
+	done := make(chan struct{})
+	go func() {
+		dl.seedAndDrop("ctx-cancel-task-id", nil, 1000)
+		close(done)
+	}()
+
+	select {
+	case <-done:
+	case <-time.After(2 * time.Second):
+		t.Fatal("seedAndDrop should return when seedCtx is cancelled")
+	}
+}