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.go

@@ -0,0 +1,105 @@
+package agent
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"sync"
+
+	"github.com/torrentclaw/unarr/internal/config"
+)
+
+// activeTasksFilePathFn is overridable for testing.
+var activeTasksFilePathFn = func() string {
+	return filepath.Join(config.DataDir(), "active-tasks.json")
+}
+
+// ActiveTaskStore persists the dispatch payloads (agent.Task) of in-flight
+// DOWNLOAD tasks so the daemon can re-submit them after a restart and have the
+// downloaders resume the partial data — torrent via the persisted
+// piece-completion DB, debrid via HTTP Range, usenet via its segment tracker.
+//
+// Distinct from LocalState (tasks.json), which holds transient status/progress
+// for syncing to the web; this holds the re-dispatch payload needed to restart
+// the work. An entry is added when a download starts and removed when it
+// reaches a genuine terminal state (completed / failed / cancelled) — but NOT
+// when the daemon is shutting down, so an interrupted download survives the
+// restart and resumes.
+type ActiveTaskStore struct {
+	mu    sync.Mutex
+	tasks map[string]Task
+}
+
+// NewActiveTaskStore creates an empty store. Call Load() to hydrate it from disk.
+func NewActiveTaskStore() *ActiveTaskStore {
+	return &ActiveTaskStore{tasks: make(map[string]Task)}
+}
+
+// Add records (or replaces) a task and persists the set.
+func (s *ActiveTaskStore) Add(t Task) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.tasks[t.ID] = t
+	s.flushLocked()
+}
+
+// Remove drops a task and persists the set. No-op if absent.
+func (s *ActiveTaskStore) Remove(taskID string) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if _, ok := s.tasks[taskID]; !ok {
+		return
+	}
+	delete(s.tasks, taskID)
+	s.flushLocked()
+}
+
+// Load reads the persisted tasks from disk into the store and returns them.
+// Returns nil on a missing or unreadable file (a fresh daemon has nothing to
+// resume). Safe to call once at startup before any Add/Remove.
+func (s *ActiveTaskStore) Load() []Task {
+	data, err := os.ReadFile(activeTasksFilePathFn())
+	if err != nil {
+		return nil
+	}
+	var tasks []Task
+	if json.Unmarshal(data, &tasks) != nil {
+		return nil
+	}
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.tasks = make(map[string]Task, len(tasks))
+	for _, t := range tasks {
+		if t.ID != "" {
+			s.tasks[t.ID] = t
+		}
+	}
+	out := make([]Task, 0, len(s.tasks))
+	for _, t := range s.tasks {
+		out = append(out, t)
+	}
+	return out
+}
+
+// flushLocked atomically writes the current set to disk. Caller holds s.mu.
+// Best-effort: a write failure is non-fatal (the in-memory set stays correct;
+// at worst a crash before the next flush loses one resume entry).
+func (s *ActiveTaskStore) flushLocked() {
+	tasks := make([]Task, 0, len(s.tasks))
+	for _, t := range s.tasks {
+		tasks = append(tasks, t)
+	}
+	data, err := json.MarshalIndent(tasks, "", "  ")
+	if err != nil {
+		return
+	}
+	path := activeTasksFilePathFn()
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+		return
+	}
+	tmp := path + ".tmp"
+	if err := os.WriteFile(tmp, data, 0o644); err != nil {
+		return
+	}
+	_ = os.Rename(tmp, path)
+}