feat(api): add price history, Stringee SMS, Zalo OA, WebSocket notifications, and feature-listing command

- Add PriceHistory model + migration, price-changed domain event, and event handler
- Add GetPriceHistory query handler and controller endpoint
- Implement StringeeSmsService and ZaloOaService with unit tests
- Add Zalo ZNS templates for Vietnamese notification messages
- Add WebSocket notification gateway for real-time push
- Add FeatureListingCommand for promoted listings
- Apply remaining consistent-type-imports lint fixes across API modules

Co-Authored-By: Paperclip <noreply@paperclip.ing>

prisma/migrations/20260416200000_add_price_history/migration.sql

@@ -0,0 +1,16 @@
+-- CreateTable: PriceHistory (listing price change tracking)
+CREATE TABLE "PriceHistory" (
+    "id" TEXT NOT NULL,
+    "listingId" TEXT NOT NULL,
+    "oldPrice" BIGINT NOT NULL,
+    "newPrice" BIGINT NOT NULL,
+    "changedAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "PriceHistory_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE INDEX "PriceHistory_listingId_changedAt_idx" ON "PriceHistory"("listingId", "changedAt" DESC);
+
+-- AddForeignKey
+ALTER TABLE "PriceHistory" ADD CONSTRAINT "PriceHistory_listingId_fkey" FOREIGN KEY ("listingId") REFERENCES "Listing"("id") ON DELETE CASCADE ON UPDATE CASCADE;

prisma/schema.prisma

@@ -333,9 +333,10 @@ model Listing {
   createdAt        DateTime        @default(now())
   updatedAt        DateTime        @updatedAt
 
-  transactions Transaction[]
-  inquiries    Inquiry[]
-  orders       Order[]
+  transactions  Transaction[]
+  inquiries     Inquiry[]
+  orders        Order[]
+  priceHistories PriceHistory[]
 
   // --- Single-column indexes ---
   @@index([status])
@@ -357,6 +358,17 @@ model Listing {
   @@index([status, transactionType, priceVND])
 }
 
+model PriceHistory {
+  id        String   @id @default(cuid())
+  listingId String
+  listing   Listing  @relation(fields: [listingId], references: [id], onDelete: Cascade)
+  oldPrice  BigInt
+  newPrice  BigInt
+  changedAt DateTime @default(now())
+
+  @@index([listingId, changedAt(sort: Desc)])
+}
+
 // =============================================================================
 // SEARCH
 // =============================================================================