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/db/migrator/migrator.go

@@ -69,10 +69,7 @@ func Migrate() {
 }
 
 func clearSystemConfigCache() {
-	if db.Redis == nil {
-		return
-	}
-	if err := db.Redis.Del(context.Background(), db.PrefixedKey(model.SystemConfigRedisHashKey)).Err(); err != nil {
+	if err := model.InvalidateAllSystemConfigCaches(context.Background()); err != nil {
 		log.Printf("[%s] clear system config cache failed: %v\n", dbType(), err)
 	}
 }