feat(db): 新增 exchange_rates 汇率表，支持联合主键与基础交叉汇率数据

src/db/schema.ts

@@ -1,4 +1,4 @@
-import { pgTable, uuid, varchar, timestamp, pgEnum, numeric } from "drizzle-orm/pg-core";
+import { pgTable, uuid, varchar, timestamp, pgEnum, numeric, uniqueIndex } from "drizzle-orm/pg-core";
 
 export const users = pgTable("users", {
   id: uuid("id").primaryKey().$defaultFn(() => crypto.randomUUID()),
@@ -51,3 +51,14 @@ export const transactions = pgTable("transactions", {
   createdAt: timestamp("created_at", { withTimezone: true, mode: "date" })
     .defaultNow(),
 });
+
+export const exchangeRates = pgTable("exchange_rates", {
+  id: uuid("id").primaryKey().$defaultFn(() => crypto.randomUUID()),
+  fromCurrency: varchar("from_currency", { length: 10 }).notNull(),
+  toCurrency: varchar("to_currency", { length: 10 }).notNull(),
+  rate: numeric("rate", { precision: 20, scale: 8 }).notNull(),
+  updatedAt: timestamp("updated_at", { withTimezone: true, mode: "date" })
+    .defaultNow(),
+}, (table) => [
+  uniqueIndex("currency_pair_idx").on(table.fromCurrency, table.toCurrency),
+]);