Add draft-only invoicing rules, send reminders, and time clock billing.

Restrict line item edits to draft invoices, auto-create drafts on clock-out,
and add sendReminderAt scheduling with dashboard due reminders.

Co-authored-by: Cursor <cursoragent@cursor.com>

drizzle/0015_invoice_send_reminder_at.sql

@@ -0,0 +1 @@
+ALTER TABLE "beenvoice_invoice" ADD COLUMN "send_reminder_at" timestamp;