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>

app/api/transactions/batch-match-invoices/route.ts

@@ -1,6 +1,6 @@
 import { createClient } from '@/lib/supabase/server'
 import { NextResponse } from 'next/server'
-import { getBestInvoiceMatch } from '@/lib/invoice/invoice-matching'
+import { getBestInvoiceMatch } from '@/lib/invoices/invoice-matching'
 import type { Transaction } from '@/types'
 
 /**