chore(backend): rename lastSyncTimestamp to since across Ping sync API for consistency

- Updated parameter name in `PingController`, `PingApi`, and related tests to align with SyncManager conventions.

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

@@ -80,9 +80,10 @@ class PingController(
 
     @GetMapping("/ping/sync")
     override suspend fun syncPings(
-        @RequestParam(required = false, defaultValue = "0") lastSyncTimestamp: Long
+        // Changed the parameter name to 'since' to match SyncManager convention
+        @RequestParam(required = false, defaultValue = "0") since: Long
     ): List<PingEvent> {
-        return pingUseCase.getPingsSince(lastSyncTimestamp).map {
+        return pingUseCase.getPingsSince(since).map {
             PingEvent(
                 id = it.id.toString(),
                 message = it.message,

backend/services/ping/ping-service/src/test/kotlin/at/mocode/ping/service/PingControllerTest.kt

@@ -153,7 +153,8 @@ class PingControllerTest {
         )
 
         // When & Then
-        val mvcResult: MvcResult = mockMvc.perform(get("/ping/sync").param("lastSyncTimestamp", timestamp.toString()))
+        // Changed parameter name to 'since' to match the controller update
+        val mvcResult: MvcResult = mockMvc.perform(get("/ping/sync").param("since", timestamp.toString()))
             .andExpect(request().asyncStarted())
             .andReturn()
 

contracts/ping-api/src/commonMain/kotlin/at/mocode/ping/api/PingApi.kt

@@ -10,5 +10,6 @@ interface PingApi {
     suspend fun securePing(): PingResponse
 
     // Phase 3: Delta-Sync
-    suspend fun syncPings(lastSyncTimestamp: Long): List<PingEvent>
+    // Changed parameter name to 'since' to match SyncManager convention and backend controller
+    suspend fun syncPings(since: Long): List<PingEvent>
 }