fix: raise default heap to 4g (2g OOMs at mainnet scale)
A full mainnet sync to tip (~317k handles, ~14.7M ada_handle_history_item rows) exhausted the 2g heap with recurring OutOfMemoryError, wedging the HTTP server while the indexer thread limped on. 4g runs comfortably (steady-state heap ~650MB) and the REST API stays responsive at tip. Surfaced by the mainnet full-sync integration-test harness. Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>