feat(accounting): intercompany netting workbench — data model (#1058)

[carbon-agent]

What

Workstream 4 (netting) data-model foundation for intercompany maturity (issue #1058), from .ai/specs/2026-07-04-intercompany-maturity.md.

Stacked on #1224 (tolerance + FX matching). Base is feature/intercompany-maturity-matching; GitHub will retarget to main when #1224 merges. Review #1224 first.

Scope: schema only, on purpose

The consuming statement-lifecycle RPCs, accounting service functions, and workbench UI reference these new tables/columns in TypeScript, which requires regenerating packages/database/src/types.ts against a migrated local DB. The build environment for this branch has no running stack, and repo policy is to not rebuild the DB autonomously (AGENTS.md: "Never rebuild the database to test changes — wait for the user"). So this PR lands the migration + seed only — every line is SQL or pure seed data, grounded in existing precedent — and the consuming code follows once types can be regenerated. This mirrors how #1224 shipped matching without new TS-referenced DB objects.

Migration 20260725143012_intercompany-netting-foundation.sql

• accountDefault.intercompanyNettingAccount (FK to account.id, by id per the control-account lesson) + a per-group 1135 Intercompany Netting Clearing current-asset account, seeded (sibling of 1130, parent resolved by isGroup+name+class, never by number) and backfilled. Left NULLABLE so an unset account makes the future settlement path raise, rather than posting to the wrong clearing account.
• intercompanyNettingStatement + intercompanyNettingStatementLine — group-level, so they follow the intercompanyTransaction precedent (single-column PK, companyGroupId + explicit company columns, currencyCode a bare TEXT with no FK because a two-company row can't reference the per-company currency(code, companyId) table).
• payment.nettingStatementId back-reference + partial index (settlement audit trail).
• ICNS-%{yyyy}- readable sequence backfilled for existing companies + seeded in seed.data.ts for new ones.
• RLS mirrors the intercompanyTransaction source-OR-target shape (accounting_create/update/delete). Line policies gate on the parent statement's two companies via EXISTS, fully-qualifying the line's statementId to disambiguate it from the statement's readable-number column of the same name (a bug I caught and fixed during review).

Verification

• pnpm exec biome check on seed.data.ts — clean.
• pnpm exec turbo run typecheck --filter=@carbon/database — passes.
• The migration cannot be applied/db:migrate'd here (no local stack); the SQL is grounded line-for-line in 20260724100357 (the matching migration) and 20260630093809 (AR/AP payments). Please apply + regenerate types on a real stack before merge — that step also produces the types.ts update every DB PR carries.

Follow-up (still needed for #1058)

• Netting statement-lifecycle RPCs (createNettingStatement / propose / agree / cancel) + settlement. Design note: settlement should ride the existing post-payment edge function (stage Draft payment + invoiceSettlement rows, invoke per company) rather than a self-contained settleNettingStatement RPC as the spec's RPC sketch implied — posting lives in the edge function, not an RPC. Resolve that gap when building settlement.
• The matrix → statement-list → statement-Drawer workbench UI, and the group IC-settings form (validator already exists, unwired).
• Workstream 3 (document mirroring) — Inngest job, cross-company writes, item-reference table, UI — entirely remaining.

Tracking spec: .ai/specs/2026-07-04-intercompany-maturity.md

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