feat(download): judge a queued slskd peer by its queue position

No bytes for ten minutes usually means a peer has queued us, and the
stall timer could not tell position 2 from position 400: the first was
abandoned while it was about to start, the second was waited on for ten
minutes for nothing.

While a requested file is "Queued, Remotely", the grab asks slskd for
its place (GET .../downloads/{user}/{id}/position, which asks the peer)
once a minute. A place that improved counts as progress and restarts
the stall clock. A place beyond 50 twice running gives the peer up at
once, and the manager moves to the next copy; two readings because
slskd documents the figure as possibly inaccurate. Waiting in a queue
has an overall ceiling of an hour without a byte, since a queue moving
one place an hour would otherwise hold the grab all day. As with a
stall, files that already arrived still go forward.

Closes #275

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017HJiuc3ZZhxsPXz3ozTirT

backend/download/provider_slskd_test.go

@@ -65,8 +65,13 @@ type slskdStub struct {
 	// under downloads when it is enqueued, keyed by file base name.
 	batches     bool
 	batchBodies []map[string]any
-	delivered   map[string]string
-	downloads   string
+
+	// positions is what the queue-position endpoint answers, in order;
+	// the last repeats.  positionAsks counts the calls.
+	positions    []int
+	positionAsks int
+	delivered    map[string]string
+	downloads    string
 }
 
 func newSlskdStub(t *testing.T) *slskdStub {
@@ -150,6 +155,23 @@ func newSlskdStub(t *testing.T) *slskdStub {
 		s.paths = append(s.paths, r.URL.EscapedPath())
 		s.mu.Unlock()
 
+		if r.Method == http.MethodGet && strings.HasSuffix(r.URL.Path, "/position") {
+			s.mu.Lock()
+			idx := min(s.positionAsks, len(s.positions)-1)
+			s.positionAsks++
+			place := 0
+
+			if idx >= 0 {
+				place = s.positions[idx]
+			}
+
+			s.mu.Unlock()
+
+			writeJSON(t, w, place)
+
+			return
+		}
+
 		if r.Method == http.MethodPost && strings.HasSuffix(r.URL.Path, "/batches") {
 			s.enqueueBatch(t, w, r)
 
@@ -379,6 +401,8 @@ func newStubSlskd(t *testing.T, stub *slskdStub) (*slskd, string) {
 	// tests about stalls and absences set their own.
 	s.stallAfter = time.Minute
 	s.absentGrace = time.Minute
+	s.positionPoll = time.Millisecond
+	s.queueCeiling = time.Hour
 
 	return s, downloads
 }