[M2] Live NEAR Lake S3 streaming with shard filtering

[Fosurero]

Description

Connect to the real NEAR Lake S3 bucket (testnet and mainnet) and stream blocks with shard filtering.

Tasks

• AWS credential configuration (env vars + config file)
• Live testnet streaming end-to-end
• Mainnet support
• Configurable S3 endpoint for custom providers
• Backfill mode from arbitrary start height
• Integration tests with real data snapshots

Acceptance Criteria

• Successfully stream 100k+ consecutive testnet blocks
• Only selected shards are processed
• Checkpoint/resume on restart