Run demo trading and allocate capital per symbol

Move running, running_changed_at and capital_usdt from the account to
each trading symbol; the migration copies the account capital to every
symbol and leaves them all stopped. Each symbol is sized against its
own capital with a 3x notional cap, and the capitals together must not
exceed account equity. Scheduling, dry-run decisions and the executor
follow the per-symbol flag. The global start/stop button is replaced by
start/stop and capital controls on each symbol's plan card.

web/src/pages/settings/index.vue

@@ -275,7 +275,7 @@ loadExchange()
         </el-descriptions-item>
         <el-descriptions-item label="状态">
           <el-tag v-if="exchange.running" type="success" size="small">
-            交易中（停止后才能修改 key）
+            有币种在交易（全部停止后才能修改 key）
           </el-tag>
           <span v-else>未运行</span>
         </el-descriptions-item>