feat(sync): implement Delta-Sync API and update clients to support offline-first workflow

Added `/ping/sync` endpoint with timestamp-based Delta-Sync functionality to efficiently support offline-first clients. Extended `PingApi` and frontend clients (`PingApiClient`, `PingApiKoinClient`) with `syncPings`. Updated repository, service, and controller logic for sync handling, including new JPA query `findByCreatedAtAfter`. Adjusted test doubles and completed unit tests for backend and frontend alignment. Documented sync approach and API usage.

backend/services/ping/ping-service/src/main/kotlin/at/mocode/ping/application/PingService.kt

@@ -8,6 +8,7 @@ import org.springframework.stereotype.Service
 import org.springframework.transaction.annotation.Transactional
 import kotlin.uuid.ExperimentalUuidApi
 import kotlin.uuid.Uuid
+import java.time.Instant
 
 /**
  * Application Service.
@@ -43,4 +44,10 @@ class PingService(
     override fun getPing(id: Uuid): Ping? {
         return repository.findById(id)
     }
+
+    @Transactional(readOnly = true)
+    override fun getPingsSince(timestamp: Long): List<Ping> {
+        val instant = Instant.ofEpochMilli(timestamp)
+        return repository.findByTimestampAfter(instant)
+    }
 }

backend/services/ping/ping-service/src/main/kotlin/at/mocode/ping/application/PingUseCase.kt

@@ -13,4 +13,5 @@ interface PingUseCase {
     fun executePing(message: String): Ping
     fun getPingHistory(): List<Ping>
     fun getPing(id: Uuid): Ping?
+    fun getPingsSince(timestamp: Long): List<Ping>
 }

backend/services/ping/ping-service/src/main/kotlin/at/mocode/ping/domain/PingRepository.kt

@@ -2,6 +2,7 @@ package at.mocode.ping.domain
 
 import kotlin.uuid.ExperimentalUuidApi
 import kotlin.uuid.Uuid
+import java.time.Instant
 
 /**
  * Secondary Port (Outbound Port).
@@ -12,4 +13,5 @@ interface PingRepository {
     fun save(ping: Ping): Ping
     fun findAll(): List<Ping>
     fun findById(id: Uuid): Ping?
+    fun findByTimestampAfter(timestamp: Instant): List<Ping>
 }

backend/services/ping/ping-service/src/main/kotlin/at/mocode/ping/infrastructure/persistence/PingRepositoryAdapter.kt

@@ -8,6 +8,7 @@ import kotlin.uuid.ExperimentalUuidApi
 import kotlin.uuid.Uuid
 import kotlin.uuid.toJavaUuid
 import kotlin.uuid.toKotlinUuid
+import java.time.Instant
 
 @OptIn(ExperimentalUuidApi::class)
 @Repository
@@ -30,6 +31,10 @@ class PingRepositoryAdapter(
         return jpaRepository.findById(id.toJavaUuid()).map { it.toDomain() }.orElse(null)
     }
 
+    override fun findByTimestampAfter(timestamp: Instant): List<Ping> {
+        return jpaRepository.findByCreatedAtAfter(timestamp).map { it.toDomain() }
+    }
+
     private fun Ping.toEntity() = PingJpaEntity(
         id = this.id.toJavaUuid(),
         message = this.message,

backend/services/ping/ping-service/src/main/kotlin/at/mocode/ping/infrastructure/persistence/SpringDataPingRepository.kt

@@ -2,5 +2,8 @@ package at.mocode.ping.infrastructure.persistence
 
 import org.springframework.data.jpa.repository.JpaRepository
 import java.util.UUID
+import java.time.Instant
 
-interface SpringDataPingRepository : JpaRepository<PingJpaEntity, UUID>
+interface SpringDataPingRepository : JpaRepository<PingJpaEntity, UUID> {
+    fun findByCreatedAtAfter(createdAt: Instant): List<PingJpaEntity>
+}

backend/services/ping/ping-service/src/main/kotlin/at/mocode/ping/infrastructure/web/PingController.kt

@@ -3,6 +3,7 @@ package at.mocode.ping.infrastructure.web
 import at.mocode.ping.api.EnhancedPingResponse
 import at.mocode.ping.api.HealthResponse
 import at.mocode.ping.api.PingApi
+import at.mocode.ping.api.PingEvent
 import at.mocode.ping.api.PingResponse
 import at.mocode.ping.application.PingUseCase
 import io.github.resilience4j.circuitbreaker.annotation.CircuitBreaker
@@ -12,6 +13,7 @@ import org.springframework.web.bind.annotation.*
 import java.time.ZoneOffset
 import java.time.format.DateTimeFormatter
 import kotlin.random.Random
+import kotlin.uuid.ExperimentalUuidApi
 
 /**
  * Driving Adapter (REST Controller).
@@ -20,6 +22,7 @@ import kotlin.random.Random
 @RestController
 // Spring requires using `originPatterns` (not wildcard `origins`) when credentials are enabled.
 @CrossOrigin(allowedHeaders = ["*"], allowCredentials = "true", originPatterns = ["*"])
+@OptIn(ExperimentalUuidApi::class)
 class PingController(
     private val pingUseCase: PingUseCase
 ) : PingApi {
@@ -75,6 +78,19 @@ class PingController(
         return createResponse(domainPing, "secure-pong")
     }
 
+    @GetMapping("/ping/sync")
+    override suspend fun syncPings(
+        @RequestParam(required = false, defaultValue = "0") lastSyncTimestamp: Long
+    ): List<PingEvent> {
+        return pingUseCase.getPingsSince(lastSyncTimestamp).map {
+            PingEvent(
+                id = it.id.toString(),
+                message = it.message,
+                lastModified = it.timestamp.toEpochMilli()
+            )
+        }
+    }
+
     // Helper
     private fun createResponse(domainPing: at.mocode.ping.domain.Ping, status: String) = PingResponse(
         status = status,