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/context.go

@@ -10,6 +10,7 @@ import (
 
 func (s *Server) handleWeekendContext(w http.ResponseWriter, _ *http.Request) {
 	if !s.hasLocalQuery() {
+		markDataResponse(w, "none", "limited")
 		writeJSON(w, query.WeekendContext{TemporalState: query.TemporalNoSeason})
 		return
 	}
@@ -29,9 +30,33 @@ func (s *Server) handleWeekendContext(w http.ResponseWriter, _ *http.Request) {
 		writeError(w, err, http.StatusInternalServerError, false)
 		return
 	}
+	if focus := focusedContextSession(context); focus != nil {
+		markDataResponse(w, focus.Availability.Source, focus.Availability.Freshness)
+	} else {
+		markDataResponse(w, "none", "limited")
+	}
 	writeJSON(w, context)
 }
 
+// focusedContextSession selects the session whose state the Weekend shell is
+// presenting. An older terminal/default session must never override an
+// upcoming focus session's metadata.
+func focusedContextSession(context query.WeekendContext) *query.ContextSession {
+	if context.ActiveSession != nil {
+		return context.ActiveSession
+	}
+	if context.FocusMeeting == nil {
+		return nil
+	}
+	focusKey := context.FocusMeeting.MeetingKey
+	for _, ref := range []*query.ContextSession{context.NextSession, context.PreviousCompletedSession, context.DefaultAnalysisSession} {
+		if ref != nil && ref.Meeting != nil && ref.Meeting.MeetingKey == focusKey {
+			return ref
+		}
+	}
+	return nil
+}
+
 func liveEvidence(data *live.LiveStreamData, active, final bool) query.LiveEvidence {
 	return query.LiveEvidence{Active: active, Final: final, MeetingName: data.Session.MeetingName, CircuitName: data.Session.CircuitName, SessionName: data.Session.SessionName, SessionType: data.Session.SessionType}
 }