[P2 MEDIUM] No `reportedInGstr1` Flag on CreditNote or OrganizationInvoice

[teetangh]

Summary

Neither CreditNote nor OrganizationInvoice has a reportedInGstr1 flag. When a GSTR-1 builder is eventually built (#1361), there is no way to track which invoices/credit notes have already been exported, risking double-reporting across months.

Priority: P2 MEDIUM

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What Exists

• TDSRecord.reportedInForm26Q — pattern exists for TDS
• CreditNote — no reporting flag
• OrganizationInvoice — no reporting flag

What's Missing

1. CreditNote.reportedInGstr1 Boolean @default(false)
2. OrganizationInvoice.reportedInGstr1 Boolean @default(false)
3. CreditNote.reportedInGstr3b Boolean @default(false) (for future GSTR-3B builder)
4. OrganizationInvoice.reportedInGstr3b Boolean @default(false)

Impact

• Double-reporting risk — GSTR-1 export could include the same invoice in multiple months
• Audit trail gap — no record of which invoices have been reported

Fix

1. Add reportedInGstr1 Boolean @default(false) to both models
2. Add reportedInGstr3b Boolean @default(false) to both models
3. Stamp flags after GSTR-1/GSTR-3B export
4. Add composite index for efficient filtering

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Related: Part of Organizational Finance Deep Dive (2026-09-03). See bugs/financial-audit/10-org-finance-deep-dive.md. Pairs with #1361.