refactor: consolidate lib/invoice/ into lib/invoices/

Merge the singular lib/invoice/ directory into the plural lib/invoices/
to align with the codebase convention (transactions/, extensions/, reports/, etc.).
Updates all import paths and CLAUDE.md architecture docs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/transactions/__tests__/ingest.test.ts

@@ -26,7 +26,7 @@ vi.mock('@/lib/bookkeeping/transaction-entries', () => ({
 }))
 
 const mockGetBestInvoiceMatch = vi.fn()
-vi.mock('@/lib/invoice/invoice-matching', () => ({
+vi.mock('@/lib/invoices/invoice-matching', () => ({
   getBestInvoiceMatch: (...args: unknown[]) => mockGetBestInvoiceMatch(...args),
 }))
 

lib/transactions/ingest.ts

@@ -1,7 +1,7 @@
 import type { SupabaseClient } from '@supabase/supabase-js'
 import { evaluateMappingRules } from '@/lib/bookkeeping/mapping-engine'
 import { createTransactionJournalEntry } from '@/lib/bookkeeping/transaction-entries'
-import { getBestInvoiceMatch } from '@/lib/invoice/invoice-matching'
+import { getBestInvoiceMatch } from '@/lib/invoices/invoice-matching'
 import type { Transaction } from '@/types'
 
 /**