Elasticsearch-compatible bulk ingest

Accepts Elasticsearch _bulk API format for zero-config migration from Filebeat, Logstash, Vector, Fluentd. Prefer /es/_bulk; /ingest/bulk is an alias to the same handler.

• _index is accepted but mapped to _source tag (LynxDB is single-index by design).
• _type is ignored.
• Response mimics ES bulk shape for client compatibility.