feat(quotes): admin decline-on-behalf with optional reason

Add a "Decline on behalf" action mirroring accept-on-behalf, for when a
customer says no by phone/email. Flips a draft/sent/expired quote to
declined, stamps declined_at, closes the public response window, and
invalidates outstanding accept/decline tokens so the emailed link can't
toggle it back. Optional free-text reason persisted to a new
quotes.decline_reason column (migration 115) and shown on the quote
detail page. Hard-delete intentionally not included.

backend/migrations/core/115_add_quote_decline_reason.js

@@ -0,0 +1,31 @@
+/**
+ * Migration: admin decline-quote reason.
+ *
+ * Background: admins can now decline a quote on the customer's behalf
+ * ("customer told us by phone they're not going ahead") instead of
+ * waiting for the public response link. This stores an optional free-text
+ * reason alongside the existing `declined_at` timestamp so the quote
+ * detail page can show WHY it was declined.
+ *
+ * Nullable, no default — existing declined rows simply carry no reason,
+ * which is exactly how customer-side declines already look. No behaviour
+ * change on upgrade.
+ *
+ * Idempotent: guarded by hasColumn so a re-run is a no-op.
+ */
+
+exports.up = async function(knex) {
+  if (!(await knex.schema.hasTable('quotes'))) return;
+  if (await knex.schema.hasColumn('quotes', 'decline_reason')) return;
+  await knex.schema.alterTable('quotes', (table) => {
+    table.text('decline_reason');
+  });
+};
+
+exports.down = async function(knex) {
+  if (!(await knex.schema.hasTable('quotes'))) return;
+  if (!(await knex.schema.hasColumn('quotes', 'decline_reason'))) return;
+  await knex.schema.alterTable('quotes', (table) => {
+    table.dropColumn('decline_reason');
+  });
+};