perf(config): add Otter RAM cache layer for system configs

Introduce pkg/cache/ram on top of existing Redis/DB config reads.
GetByKey now checks local RAM before Redis. Unified invalidation clears
RAM and Redis hash fields on admin writes, with pub/sub for multi-node
RAM eviction. Add tests and wire create/update/migrator write paths.

internal/testhelper/test_helper.go

@@ -75,6 +75,7 @@ func SetupTestEnvironment(t *testing.T) (*gorm.DB, *miniredis.Miniredis, func())
 
 	// Cleanup function
 	cleanup := func() {
+		model.ResetSystemConfigRAMCacheForTest()
 		_ = redisClient.Close()
 		mr.Close()
 		// Reset database and Redis references