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/api/cache.go

@@ -164,8 +164,8 @@ func (c *Cache) Get(key string) ([]byte, bool) {
 	if ttl > 0 {
 		age := time.Since(time.Unix(createdAt, 0))
 		if age > ttl {
-			// Expired — delete and return miss.
-			_, _ = c.db.Exec(`DELETE FROM cache WHERE key = ?`, key)
+			// Expired entries remain stored so get() can use them as a stale
+			// fallback if the live request fails. Prune() owns physical cleanup.
 			atomic.AddInt64(&c.stats.Misses, 1)
 			return nil, false
 		}