upwatch: hold a seeding torrent app for one line test, then put its limit back exactly (R-1250)

When the agent waits for quiet and qBittorrent or Transmission runs: record the limit first,
hold at 1 KiB/s, ask the agent for a test on request, put the old limit back (deferred; a
household change during the test stays). Start-up puts back a limit a killed controller left;
the 50 % rule skips while a hold record exists. Commit only.

controller/internal/agentapi/linetest.go

@@ -16,6 +16,37 @@ type LineTestState struct {
 	Mbps       float64 `json:"mbps,omitempty"`
 	MeasuredAt string  `json:"measured_at,omitempty"` // RFC3339 UTC
 	Method     string  `json:"method,omitempty"`
+	// WaitingForQuiet (agent ≥ 0.165.0, R-1250): a test is due and could start now, but the host's uplink has not been
+	// quiet for 10 minutes. With a seeding torrent app the controller then holds the app's limit and calls LineTestRun.
+	WaitingForQuiet bool `json:"waiting_for_quiet,omitempty"`
+}
+
+// LineTestRunResult mirrors POST /host/line-test/run. OK=false with Refused: nothing ran (not-due, outside-window,
+// tries-used, no-tunnel, heavy-op, not-quiet, server-busy); OK=false without it: the runs failed.
+type LineTestRunResult struct {
+	OK       bool      `json:"ok"`
+	Mbps     float64   `json:"mbps,omitempty"`
+	RunsMbps []float64 `json:"runs_mbps,omitempty"`
+	Refused  string    `json:"refused,omitempty"`
+	Error    string    `json:"error,omitempty"`
+}
+
+// LineTestRunTimeout bounds the synchronous request: about 55 s on the agent (5 s settle, 10 s quiet check, three
+// 10-second runs 5 s apart), each run itself bounded at 25 s.
+const LineTestRunTimeout = 2 * time.Minute
+
+// LineTestRun calls POST /host/line-test/run (R-1250): a test on request while the caller holds its torrent app's
+// upload down. An agent before the route answers 404 (*StatusError).
+func (c *Client) LineTestRun(ctx context.Context) (LineTestRunResult, error) {
+	var out LineTestRunResult
+	body, err := c.postSlow(ctx, "/host/line-test/run", struct{}{}, LineTestRunTimeout)
+	if err != nil {
+		return out, err
+	}
+	if err := json.Unmarshal(body, &out); err != nil {
+		return out, fmt.Errorf("agentapi: decode /host/line-test/run: %w", err)
+	}
+	return out, nil
 }
 
 // LineTest calls GET /host/line-test.