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/errs.go

@@ -4,7 +4,8 @@
 package db
 
 const (
-	errRedisHashSetFailed  = "failed to set redis hash: %w"
+	errRedisHashSetFailed    = "failed to set redis hash: %w"
+	errRedisHashDeleteFailed = "failed to delete redis hash field: %w"
 	errUnmarshalDataFailed = "failed to unmarshal data: %w"
 	errMarshalDataFailed   = "failed to marshal data: %w"
 	errRedisKeySetFailed   = "failed to set redis key: %w"