feat: migrate monthly city trade rates

app/game-engine/src/turn/databaseHooks.ts

@@ -261,9 +261,9 @@ const buildCityUpdate = (
     if (trust !== null) {
         data.trust = trust;
     }
-    if (trade !== null) {
-        data.trade = trade;
-    }
+    // trade가 없는 도시는 레거시 DB에서 NULL이다. City meta에 trade가
+    // 없다는 사실도 dirty city의 전체 snapshot 일부로 영속화한다.
+    data.trade = trade;
     if (region !== null) {
         data.region = region;
     }

app/game-engine/src/turn/monthlyEventHandler.ts

@@ -1,3 +1,6 @@
+import { LiteHashDRBG, RandUtil } from '@sammo-ts/common';
+import { simpleSerialize } from '@sammo-ts/logic/war/utils.js';
+
 import type { InMemoryTurnWorld, TurnCalendarContext, TurnCalendarHandler } from './inMemoryWorld.js';
 import type { TurnEvent } from './types.js';
 
@@ -18,6 +21,50 @@ export type MonthlyEventActionHandler = (
 export type MonthlyEventActionRegistry = ReadonlyMap<string, MonthlyEventActionHandler>;
 
 const COMPARATORS = new Set(['==', '!=', '<', '>', '<=', '>=']);
+const CITY_TRADE_PROBABILITY_BY_LEVEL: Readonly<Record<number, number>> = {
+    1: 0,
+    2: 0,
+    3: 0,
+    4: 0.2,
+    5: 0.4,
+    6: 0.6,
+    7: 0.8,
+    8: 1,
+};
+
+const resolveHiddenSeed = (world: InMemoryTurnWorld): string | number => {
+    const state = world.getState();
+    const rawSeed = state.meta.hiddenSeed ?? state.meta.seed ?? state.id;
+    return typeof rawSeed === 'string' || typeof rawSeed === 'number' ? rawSeed : String(rawSeed);
+};
+
+export const createRandomizeCityTradeRateHandler = (options: {
+    getWorld: () => InMemoryTurnWorld | null;
+}): MonthlyEventActionHandler => {
+    return (_args, environment) => {
+        const world = options.getWorld();
+        if (!world) {
+            return;
+        }
+        const rng = new RandUtil(
+            new LiteHashDRBG(
+                simpleSerialize(resolveHiddenSeed(world), 'randomizeCityTradeRate', environment.year, environment.month)
+            )
+        );
+
+        for (const city of world.listCities()) {
+            const probability = CITY_TRADE_PROBABILITY_BY_LEVEL[city.level];
+            if (probability === undefined) {
+                throw new Error(`Unsupported city level for RandomizeCityTradeRate: ${city.level} (cityId=${city.id})`);
+            }
+            const trade = probability > 0 && rng.nextBool(probability) ? rng.nextRangeInt(95, 105) : null;
+            const { trade: _previousTrade, ...metaWithoutTrade } = city.meta;
+            world.updateCity(city.id, {
+                meta: trade === null ? metaWithoutTrade : { ...metaWithoutTrade, trade },
+            });
+        }
+    };
+};
 
 const compare = (left: readonly (number | null)[], operator: string, right: readonly (number | null)[]): boolean => {
     const normalize = (values: readonly (number | null)[]): string =>

app/game-engine/src/turn/turnDaemon.ts

@@ -34,7 +34,11 @@ import { createNeutralAuctionRegistrar } from '../auction/neutralRegistrar.js';
 import { createTournamentRewardFinalizer } from '../tournament/finalizer.js';
 import { createTournamentAutoStartHandler } from './tournamentAutoStart.js';
 import { createYearbookHandler } from './yearbookHandler.js';
-import { createMonthlyEventHandler, type MonthlyEventActionHandler } from './monthlyEventHandler.js';
+import {
+    createMonthlyEventHandler,
+    createRandomizeCityTradeRateHandler,
+    type MonthlyEventActionHandler,
+} from './monthlyEventHandler.js';
 import { DatabaseTurnDaemonLease, TurnDaemonLeaseUnavailableError } from '../lifecycle/databaseTurnDaemonLease.js';
 
 export interface TurnDaemonRuntimeOptions {
@@ -142,6 +146,12 @@ const createTurnDaemonRuntimeWithLease = async (
                 event.action.some((action) => Array.isArray(action) && action[0] === name)
         );
     const eventActions = new Map<string, MonthlyEventActionHandler>();
+    eventActions.set(
+        'RandomizeCityTradeRate',
+        createRandomizeCityTradeRateHandler({
+            getWorld: () => worldRef,
+        })
+    );
     eventActions.set('ProcessIncome', (_args, environment) => {
         void incomeHandler.onMonthChanged?.({
             previousYear: environment.month === 1 ? environment.year - 1 : environment.year,

app/game-engine/src/turn/worldLoader.ts

@@ -246,7 +246,7 @@ const mapCityRow = (row: TurnEngineCityRow): City => {
         meta: {
             ...meta,
             trust: row.trust,
-            trade: row.trade,
+            ...(row.trade === null ? {} : { trade: row.trade }),
             region: row.region,
         },
     };