feat(agent): auto-resume interrupted downloads after a daemon restart

A daemon restart used to abandon in-flight downloads: the in-memory queue was
lost and the web doesn't re-dispatch a stuck task, so the user had to retry
manually. The bytes already persisted (mmap + anacrolix's piece-completion DB
keyed by info_hash; debrid via Range; usenet via its tracker) — the daemon just
didn't re-attempt the work.

ActiveTaskStore persists each in-flight download's agent.Task payload to
active-tasks.json; the daemon re-submits them on startup so the downloaders
resume the partial data. manager.Submit now dedups (the startup re-submit and a
later web re-dispatch can't both run), and recordFinished removes a task from
the store only on a genuine terminal — shuttingDown (set before Shutdown cancels
the task contexts) keeps shutdown-interrupted tasks so they resume next start.
Stream/seed/upgrade tasks aren't persisted; ForceStart is cleared on resume.

internal/agent/active_tasks_test.go

@@ -0,0 +1,75 @@
+package agent
+
+import (
+	"path/filepath"
+	"testing"
+)
+
+// withTempStorePath points the store file at a temp location for the duration
+// of a test and restores the original afterward.
+func withTempStorePath(t *testing.T) {
+	t.Helper()
+	orig := activeTasksFilePathFn
+	path := filepath.Join(t.TempDir(), "active-tasks.json")
+	activeTasksFilePathFn = func() string { return path }
+	t.Cleanup(func() { activeTasksFilePathFn = orig })
+}
+
+func TestActiveTaskStore_AddLoadRoundTrip(t *testing.T) {
+	withTempStorePath(t)
+
+	s := NewActiveTaskStore()
+	s.Add(Task{ID: "a", InfoHash: "hashA", Title: "Movie A", Mode: "download"})
+	s.Add(Task{ID: "b", NzbID: "nzbB", Title: "Show B"})
+
+	// A fresh store hydrated from disk must see both.
+	loaded := NewActiveTaskStore().Load()
+	if len(loaded) != 2 {
+		t.Fatalf("Load returned %d tasks, want 2", len(loaded))
+	}
+	byID := map[string]Task{}
+	for _, tk := range loaded {
+		byID[tk.ID] = tk
+	}
+	if byID["a"].InfoHash != "hashA" || byID["a"].Title != "Movie A" {
+		t.Errorf("task a not round-tripped: %+v", byID["a"])
+	}
+	if byID["b"].NzbID != "nzbB" {
+		t.Errorf("task b not round-tripped: %+v", byID["b"])
+	}
+}
+
+func TestActiveTaskStore_Remove(t *testing.T) {
+	withTempStorePath(t)
+
+	s := NewActiveTaskStore()
+	s.Add(Task{ID: "a", Title: "A"})
+	s.Add(Task{ID: "b", Title: "B"})
+	s.Remove("a")
+	s.Remove("missing") // no-op
+
+	loaded := NewActiveTaskStore().Load()
+	if len(loaded) != 1 || loaded[0].ID != "b" {
+		t.Fatalf("after Remove(a), Load = %+v, want only b", loaded)
+	}
+}
+
+func TestActiveTaskStore_Overwrite(t *testing.T) {
+	withTempStorePath(t)
+
+	s := NewActiveTaskStore()
+	s.Add(Task{ID: "a", Title: "old"})
+	s.Add(Task{ID: "a", Title: "new"}) // same id replaces
+
+	loaded := NewActiveTaskStore().Load()
+	if len(loaded) != 1 || loaded[0].Title != "new" {
+		t.Fatalf("overwrite failed: %+v", loaded)
+	}
+}
+
+func TestActiveTaskStore_LoadMissingFile(t *testing.T) {
+	withTempStorePath(t) // temp dir, no file written yet
+	if got := NewActiveTaskStore().Load(); got != nil {
+		t.Errorf("Load on missing file = %+v, want nil", got)
+	}
+}