feat(#76): harvest request-scoped availability and freshness truth

Backend-only re-cut of the #76 availability work onto main, stacked on the
canonical Weekend Context API. Adds request-scoped freshness reporting so
aggregate responses cannot report fresh when a component is stale, plus
local-first driver summary resolution and cache/pacing truth.

The frontend half of #76 is deliberately excluded: it is built on the
Weekend shell that failed owner review, including the full-width Partial
banner treatment. Availability presentation is re-cut with the shell in #89.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/web/racehub.go

@@ -17,6 +17,7 @@ func (s *Server) handleRaceHub(w http.ResponseWriter, r *http.Request) {
 	}
 
 	if !s.hasLocalQuery() {
+		markDataResponse(w, "none", "limited")
 		writeJSON(w, emptyRaceHub(sessionKey))
 		return
 	}
@@ -26,6 +27,14 @@ func (s *Server) handleRaceHub(w http.ResponseWriter, r *http.Request) {
 		writeError(w, err, http.StatusInternalServerError, false)
 		return
 	}
+	switch hub.Source {
+	case query.ResponseSourceNone:
+		markDataResponse(w, "none", "limited")
+	case query.ResponseSourcePartial:
+		markLocalResponse(w, true)
+	default:
+		markLocalResponse(w, false)
+	}
 	writeJSON(w, hub)
 }