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

[Iker2522]

Closes #278

Implements isPendingInvoice(invoice) in a dedicated, conservative utility so dashboard stats classify pending invoices consistently across both storage backends.

Changes

• backend/src/utils/stats-pending-filter.ts (new) — pure helper: only the exact literal status 'PENDING' counts as pending. Missing/null invoice, missing/empty/unknown status, and non-exact-case matches all correctly return false.
• backend/tests/stats-pending-filter.test.ts (new) — fixture-backed unit tests covering PENDING, PAID, EXPIRED, CANCELLED, unknown status, empty string, missing status, null status, case mismatch, whitespace, and missing invoice (null/undefined).
• backend/tests/fixtures/stats-pending-filter.fixture.ts (new) — named test cases for the above.
• backend/src/storage/invoice-stats.ts — wired isPendingInvoice into calculateInvoiceStats, replacing the inline status === 'PENDING' check (minimal diff: 2 lines).

Verification

• PR touches exactly the 4 paths listed in the issue
• npx tsc --noEmit passes
• Full backend suite: 177/177 tests passing (30 suites), including 15 new tests for this helper
• English-only strings and comments.

[Iker2522]

Submitting my own implementation as the officially assigned contributor for this issue (assigned via GrantFox).

• backend/src/utils/stats-pending-filter.ts — isPendingInvoice(invoice), conservative: only the exact literal 'PENDING' counts as pending; null/undefined invoice, missing/empty/unknown status, and case mismatches all return false.
• backend/tests/stats-pending-filter.test.ts + backend/tests/fixtures/stats-pending-filter.fixture.ts — 15 fixture-backed tests covering all four known statuses plus edge cases (missing/null/empty status, case sensitivity, whitespace, missing invoice).
• backend/src/storage/invoice-stats.ts — wired with a minimal 2-line diff.

Touches exactly the 4 paths listed in the issue. Full backend suite passes: 177/177 tests, 30 suites.

I know there are other PRs open for this issue (#307, #343) from before the GrantFox assignment went through — happy to defer to whichever the maintainers prefer, just wanted my implementation available for review as well.