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/memory/memory.go

@@ -108,6 +108,15 @@ func (c *Cache) Shutdown() {
 	}
 }
 
+// Close implements io.Closer so the cache layer (cache.Shutdown) can stop the
+// cleanup goroutine for the in-memory backend, matching the Redis backend
+// which closes its connection pool. Idempotent: cancelling the context does
+// not render the cache unusable, it only stops the background cleaner.
+func (c *Cache) Close() error {
+	c.Shutdown()
+	return nil
+}
+
 func (c *Cache) Set(key string, value []byte, ttl time.Duration) {
 	// Calculate the memory size of this entry
 	entrySize := int64(len(key) + len(value) + approxEntryOverhead)