Add expense invoice fields, sales tables, and Paperless env vars

Expenses now have invoiceFileUrl, invoiceFileName, paperlessUrl,
paperlessDocumentId for supplier invoice attachment.

New expense_packages junction links expenses to multiple packages.

New sales + sale_line_items + sale_packages tables for income tracking
with per-line tax rate and per-sale withholding rate.

Added saleStatusEnum and 4 audit events: expense_invoice_uploaded,
sale_created, sale_confirmed, sale_voided.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.env.example

@@ -21,3 +21,7 @@ BODY_SIZE_LIMIT=26214400
 
 # Company Links favicon fetching (set false to disable outbound fetches in offline dev)
 FAVICON_FETCH_ENABLED=true
+
+# Paperless-ngx integration (optional — leave blank to disable)
+PAPERLESS_URL=
+PAPERLESS_TOKEN=