add: processing controls — start/hold, pause/resume, retry

Three lifecycle controls on the dashboard, sharing one /api surface and the
status feed:

- Start/hold gate (#11): the watcher tracks the queue but holds processing
  until POST /api/start. Held by default; AV1DAE_AUTOSTART=1 restores start-
  on-boot. NOTE: flips the previous auto-start default.
- Pause/resume the active encode (#10): SIGSTOP/SIGCONT on the ffmpeg process
  (libsvtav1 is in-process, so one signal suspends all its threads). Resumes
  exactly where it left off; the tracker excludes paused time from elapsed.
- Retry a failed file (#3): POST /api/retry?file=NAME moves it from failed/
  back to input/, with a base-name guard against path traversal.

/status now reports running + the failed list; snapshots carry a paused flag.
Verified live: held queue, retry move, traversal -> 400, and a real encode
suspending to process state T on pause and S on resume.

Closes #3
Closes #10
Closes #11

internal/status/status.go

@@ -43,16 +43,19 @@ type Stream struct {
 // Tracker holds live progress for the active job. Safe for concurrent use:
 // the encode goroutine writes, HTTP/log readers call Snapshot.
 type Tracker struct {
-	mu        sync.RWMutex
-	file      string
-	phase     string
-	meta      *JobMeta
-	streams   []Stream
-	totalSec  float64 // source duration; 0 until known
-	outTime   float64 // encoded position in seconds
-	fps       float64
-	speed     float64
-	startedAt time.Time
+	mu          sync.RWMutex
+	file        string
+	phase       string
+	meta        *JobMeta
+	streams     []Stream
+	totalSec    float64 // source duration; 0 until known
+	outTime     float64 // encoded position in seconds
+	fps         float64
+	speed       float64
+	startedAt   time.Time
+	paused      bool
+	pausedAt    time.Time     // when the current pause began
+	pausedTotal time.Duration // accumulated paused time this job
 }
 
 // Snapshot is an immutable view of the tracker for readers.
@@ -64,6 +67,7 @@ type Snapshot struct {
 	Percent    float64   `json:"percent"`
 	FPS        float64   `json:"fps"`
 	Speed      float64   `json:"speed"`
+	Paused     bool      `json:"paused"`
 	ElapsedSec int       `json:"elapsed_sec"`
 	ETASec     int       `json:"eta_sec"`
 	StartedAt  time.Time `json:"started_at"`
@@ -83,6 +87,26 @@ func (t *Tracker) Begin(file string) {
 	t.streams = nil
 	t.totalSec, t.outTime, t.fps, t.speed = 0, 0, 0, 0
 	t.startedAt = time.Now()
+	t.paused = false
+	t.pausedAt = time.Time{}
+	t.pausedTotal = 0
+}
+
+// SetPaused records pause/resume transitions so elapsed time excludes the
+// paused span. Idempotent on repeated same-state calls.
+func (t *Tracker) SetPaused(p bool) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	if p == t.paused {
+		return
+	}
+	if p {
+		t.pausedAt = time.Now()
+	} else if !t.pausedAt.IsZero() {
+		t.pausedTotal += time.Since(t.pausedAt)
+		t.pausedAt = time.Time{}
+	}
+	t.paused = p
 }
 
 func (t *Tracker) SetPhase(p string) {
@@ -129,6 +153,9 @@ func (t *Tracker) Idle() {
 	t.streams = nil
 	t.totalSec, t.outTime, t.fps, t.speed = 0, 0, 0, 0
 	t.startedAt = time.Time{}
+	t.paused = false
+	t.pausedAt = time.Time{}
+	t.pausedTotal = 0
 }
 
 func (t *Tracker) Snapshot() Snapshot {
@@ -141,10 +168,18 @@ func (t *Tracker) Snapshot() Snapshot {
 		Streams:   t.streams,
 		FPS:       t.fps,
 		Speed:     t.speed,
+		Paused:    t.paused,
 		StartedAt: t.startedAt,
 	}
 	if !t.startedAt.IsZero() {
-		s.ElapsedSec = int(time.Since(t.startedAt).Seconds())
+		elapsed := time.Since(t.startedAt) - t.pausedTotal
+		if t.paused && !t.pausedAt.IsZero() {
+			elapsed -= time.Since(t.pausedAt)
+		}
+		if elapsed < 0 {
+			elapsed = 0
+		}
+		s.ElapsedSec = int(elapsed.Seconds())
 	}
 	if t.totalSec > 0 {
 		s.Percent = t.outTime / t.totalSec * 100