Optimize net deposit raw balance query

[emmazzz]

Summary

• Make the /get_net_deposits raw balance query start from requested asset cutoffs and use a lateral lookup into balances.
• Avoid the planner shape that sequentially scans the entire balances table before filtering to the requested assets.

Key decisions

• Keep the endpoint semantics unchanged: materialized view totals are combined with raw balance rows from each asset cutoff to the requested timestamp.
• Use LEFT JOIN LATERAL so assets with no raw rows still remain in the result and contribute zero via COALESCE.

Test plan

• cargo fmt -p deepbook-server
• cargo build -p deepbook-server
• git diff --check origin/main..HEAD

[emmazzz]

@claude review this