feat: add useBalancedRanking flag to round config schema

Defaults to true so existing rounds preserve current behavior; toggled
per-round from the ranking dashboard side panel.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit/round-config-balance-toggle.test.ts

@@ -0,0 +1,37 @@
+import { afterAll, beforeAll, describe, expect, it } from 'vitest'
+import { prisma, createCaller } from '../setup'
+import {
+  createTestUser, createTestProgram, createTestCompetition, createTestRound,
+  cleanupTestData, uid,
+} from '../helpers'
+import { roundRouter } from '../../src/server/routers/round'
+
+describe('Round.configJson.useBalancedRanking', () => {
+  let programId: string
+  let admin: { id: string; email: string; role: 'SUPER_ADMIN' }
+  const userIds: string[] = []
+
+  beforeAll(async () => {
+    const program = await createTestProgram({ name: `bal-toggle-${uid()}` })
+    programId = program.id
+    const adminUser = await createTestUser('SUPER_ADMIN')
+    userIds.push(adminUser.id)
+    admin = { id: adminUser.id, email: adminUser.email, role: 'SUPER_ADMIN' }
+  })
+
+  afterAll(async () => {
+    await cleanupTestData(programId, userIds)
+  })
+
+  it('persists useBalancedRanking via round.update', async () => {
+    const competition = await createTestCompetition(programId)
+    const round = await createTestRound(competition.id)
+    const caller = createCaller(roundRouter, admin)
+    await caller.update({
+      id: round.id,
+      configJson: { useBalancedRanking: false },
+    })
+    const reloaded = await prisma.round.findUniqueOrThrow({ where: { id: round.id } })
+    expect((reloaded.configJson as Record<string, unknown>).useBalancedRanking).toBe(false)
+  })
+})