linetest: a test on request for a box that seeds all day (R-1250)

GET /host/line-test reports waiting_for_quiet (due, in the window, tunnel up, only the quiet
missing); POST /host/line-test/run runs one measurement with a 5 s settle and a 10 s quiet
check instead of 10 quiet minutes, runs 5 s apart (~55 s). Every other weekly rule holds.
Commit only; the release rides the next train.

internal/localapi/server.go

@@ -577,6 +577,7 @@ func (s *Server) Handler() http.Handler {
 	// R-1247: the weekly upload line test's last measurement — the controller's qBittorrent limit and upload watch
 	// read it. Read-only; never measured (or the job not wired) = present:false.
 	mux.HandleFunc("GET /host/line-test", s.withGuest(s.handleLineTest))
+	mux.HandleFunc("POST /host/line-test/run", s.withGuest(s.handleLineTestRun)) // R-1250: a test on request
 	// Disk management (slice 8C) — self-scoped; format routes through the data-bearing classifier+gate.
 	mux.HandleFunc("GET /disks", s.withGuest(s.handleDisks))
 	mux.HandleFunc("GET /disks/candidates", s.withGuest(s.handleDiskCandidates))