fix(cache): make in-memory backend Close-able so Shutdown stops its cleaner

cache.Shutdown only called Close() on an io.Closer, which only the Redis
backend (CacheWrapper) satisfied -- the in-memory Cache's Shutdown
(cancels ctx, stopping its cleanupRoutine background ticker) was never
reached, so that goroutine leaked until process exit (and with the
default set-up, and whenever Redis is unreachable and EnableCache falls
back to memory).

Add Close() error to Cache (calls Shutdown, idempotent, leaves the
cache usable) so cache.Shutdown handles both backends symmetrically.
Update Shutdown doc; add test TestMemoryCacheClose_IdempotentAndStillUsable.

cache/cache.go

@@ -169,7 +169,7 @@ func EnableCache(cfg *CacheConfig) {
 }
 
 // Shutdown closes the underlying cache client if it owns external resources
-// (the Redis connection pool). No-op for in-memory caches.
+// (the Redis connection pool, or the in-memory cleanup goroutine).
 func Shutdown() {
 	if config != nil && config.Client != nil {
 		if closer, ok := config.Client.(io.Closer); ok {