Adds engine.compute_partner_grouped(period, account_type=...) that
returns per-partner aggregations with aging buckets (current/1-30/
31-60/61-90/90+). SQL-direct for performance — single GROUP BY query
with conditional sum per bucket.
Foundation for the 3 partner-grouped reports landing in commit 3:
Aged Receivable, Aged Payable, Partner Ledger.
Made-with: Cursor