Add SSE for Razorpay QR events

src/main/kotlin/com/android/trisolarisserver/component/RazorpayQrEvents.kt

@@ -0,0 +1,48 @@
+package com.android.trisolarisserver.component
+
+import com.android.trisolarisserver.controller.dto.RazorpayQrEventResponse
+import com.android.trisolarisserver.repo.RazorpayQrRequestRepo
+import org.springframework.scheduling.annotation.Scheduled
+import org.springframework.stereotype.Component
+import org.springframework.web.servlet.mvc.method.annotation.SseEmitter
+import java.time.OffsetDateTime
+import java.util.UUID
+import java.util.concurrent.ConcurrentHashMap
+
+@Component
+class RazorpayQrEvents(
+    private val qrRequestRepo: RazorpayQrRequestRepo
+) {
+    private val latestEvents: MutableMap<QrKey, RazorpayQrEventResponse> = ConcurrentHashMap()
+    private val hub = SseHub<QrKey>("qr") { key ->
+        latestEvents[key] ?: run {
+            val latest = qrRequestRepo.findTopByQrIdOrderByCreatedAtDesc(key.qrId)
+            RazorpayQrEventResponse(
+                event = "snapshot",
+                qrId = key.qrId,
+                status = latest?.status,
+                receivedAt = (latest?.createdAt ?: OffsetDateTime.now()).toString()
+            )
+        }
+    }
+
+    fun subscribe(propertyId: UUID, qrId: String): SseEmitter {
+        return hub.subscribe(QrKey(propertyId, qrId))
+    }
+
+    fun emit(propertyId: UUID, qrId: String, event: RazorpayQrEventResponse) {
+        val key = QrKey(propertyId, qrId)
+        latestEvents[key] = event
+        hub.emit(key)
+    }
+
+    @Scheduled(fixedDelayString = "25000")
+    fun heartbeat() {
+        hub.heartbeat()
+    }
+}
+
+private data class QrKey(
+    val propertyId: UUID,
+    val qrId: String
+)