feat(backend): add pending invoice stats helper (#278)

[s6pa1rta3n-lab]

Summary

Implements isPendingInvoice(invoice) in backend/src/utils/stats-pending-filter.ts to ensure consistent pending invoice classification across dashboard stats and storage aggregators, and wires it into calculateInvoiceStats in backend/src/storage/invoice-stats.ts.

Changes

• backend/src/utils/stats-pending-filter.ts: Implemented isPendingInvoice() predicate and PENDING_STATUS constant.
• backend/tests/fixtures/stats-pending-filter.fixture.ts: Added fixture cases covering valid pending invoices, non-pending statuses, and malformed inputs.
• backend/tests/stats-pending-filter.test.ts: Added unit test suite covering constants, valid fixtures, non-pending fixtures, malformed inputs, and array filtering operations.
• backend/src/storage/invoice-stats.ts: Imported isPendingInvoice and replaced inline status check with the shared predicate.

Verification

• npm test: All backend unit & isolated tests pass cleanly (187 passing tests).
• npm run typecheck: TypeScript verification passes with 0 errors.
• PR touches exactly 4 paths per issue specification.

Closes #278

Payout Routing

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