Add invoice linking on expenses: optional FK, dropdown on add form, clickable chip

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

src/lib/server/db/schema.ts

@@ -130,6 +130,9 @@ export const expenses = pgTable(
 		accountId: uuid('account_id').references((): any => companyAccounts.id, {
 			onDelete: 'set null'
 		}),
+		invoiceId: uuid('invoice_id').references((): any => invoices.id, {
+			onDelete: 'set null'
+		}),
 		submittedBy: text('submitted_by')
 			.notNull()
 			.references(() => users.id),