Deepen all business modules: methods, validations, workflows

Account:
- action_post: partner validation, line count check, sequence number
  assignment (JOURNAL/YYYY/NNNN format)
- action_register_payment: opens payment wizard from invoice
- remove_move_reconcile: undo reconciliation, reset residuals
- Register Payment button in invoice form (visible when posted+unpaid)

Sale:
- action_cancel: cancels linked draft invoices + SO state
- action_draft: reset cancelled SO to draft
- action_view_invoice: navigate to linked invoices
- Cancel/Reset buttons in form view header

Purchase:
- button_draft: reset cancelled PO to draft
- action_create_bill already existed

Stock:
- action_cancel on picking: cancels moves + picking state

CRM:
- action_set_won_rainbowman: sets Won stage + rainbow effect
- convert_opportunity: lead→opportunity type switch

HR:
- hr.contract model (name, employee, wage, dates, state)

Project:
- action_blocked on task (kanban_state)
- Task stage seed data (New, In Progress, Done, Cancelled)

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

addons/purchase/models/purchase_order.go

@@ -160,6 +160,16 @@ func initPurchaseOrder() {
 		return true, nil
 	})
 
+	// button_draft: Reset a cancelled PO back to draft (RFQ).
+	// Mirrors: odoo/addons/purchase/models/purchase_order.py PurchaseOrder.button_draft()
+	m.RegisterMethod("button_draft", func(rs *orm.Recordset, args ...interface{}) (interface{}, error) {
+		env := rs.Env()
+		for _, poID := range rs.IDs() {
+			env.Tx().Exec(env.Ctx(), `UPDATE purchase_order SET state = 'draft' WHERE id = $1 AND state = 'cancel'`, poID)
+		}
+		return true, nil
+	})
+
 	// action_create_bill: Generate a vendor bill (account.move in_invoice) from a confirmed PO.
 	// Mirrors: odoo/addons/purchase/models/purchase_order.py PurchaseOrder.action_create_invoice()
 	m.RegisterMethod("action_create_bill", func(rs *orm.Recordset, args ...interface{}) (interface{}, error) {