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/client.go

@@ -1291,6 +1291,18 @@ func (c *Client) get(ctx context.Context, path string) (json.RawMessage, error)
 // post issues an authenticated JSON POST and unwraps the {ok,data,error} envelope. The agent
 // returns 200 or 202 for accepted requests.
 func (c *Client) post(ctx context.Context, path string, body any) (json.RawMessage, error) {
+	return c.postWith(c.hc, ctx, path, body)
+}
+
+// postSlow is post with its own overall timeout, for the few synchronous routes that outlast the client's 15 s
+// (POST /host/line-test/run, about 55 s). It shares the pinned transport.
+func (c *Client) postSlow(ctx context.Context, path string, body any, timeout time.Duration) (json.RawMessage, error) {
+	hc := *c.hc
+	hc.Timeout = timeout
+	return c.postWith(&hc, ctx, path, body)
+}
+
+func (c *Client) postWith(hc *http.Client, ctx context.Context, path string, body any) (json.RawMessage, error) {
 	buf, err := json.Marshal(body)
 	if err != nil {
 		return nil, err
@@ -1302,7 +1314,7 @@ func (c *Client) post(ctx context.Context, path string, body any) (json.RawMessa
 	req.Header.Set("Authorization", "Bearer "+c.token)
 	req.Header.Set("Content-Type", "application/json")
 	start := time.Now()
-	resp, err := c.hc.Do(req)
+	resp, err := hc.Do(req)
 	if err != nil {
 		logx.Debugf(c.logger, "[agentapi] POST %s failed after %dms: %v", path, time.Since(start).Milliseconds(), err)
 		return nil, fmt.Errorf("agentapi: POST %s: %w", path, err)