v0.185.1 — E-2: the offer endpoints were mounted where nothing routed to them

Registered as /api/backup-target inside ServeStorageAPI, which main.go mounts ONLY
at /api/storage/. Live result: endpoint not found, while every unit test passed --
the tests called the handlers directly and never travelled the mount. Caught by
the first live call, which is why the live call is part of the procedure.

Moved to /api/storage/backup-target[/assign]. A new test asserts the dispatcher
source contains both paths, so a handler nothing routes to fails the suite --
the seam-wiring rule applied to a route rather than a button.

controller/internal/web/backup_target_offer.go

@@ -136,7 +136,7 @@ func degradedMessageFor(st BackupTargetState) string {
 	return backupTargetDegradedText
 }
 
-// handleBackupTargetState serves GET /api/backup-target — the dashboard's source for the degraded
+// handleBackupTargetState serves GET /api/storage/backup-target — the dashboard's source for the degraded
 // banner and the offer.
 func (s *Server) handleBackupTargetState(w http.ResponseWriter, r *http.Request) {
 	st := s.resolveBackupTargetState(r.Context())
@@ -157,7 +157,7 @@ func (s *Server) handleBackupTargetState(w http.ResponseWriter, r *http.Request)
 	writeDiskJSON(w, http.StatusOK, true, "", out)
 }
 
-// handleBackupTargetAssign is the ACCEPTANCE of the offer — POST /api/backup-target/assign.
+// handleBackupTargetAssign is the ACCEPTANCE of the offer — POST /api/storage/backup-target/assign.
 //
 // NOTHING calls this except an explicit customer action. Registration does not, the drive-gate does
 // not, and no scheduler does (E-2 §3: a drive never acquires a role by appearing). Declining is