add method "getContentsIds"

exposed/src/main/kotlin/com/insanusmokrassar/postssystem/core/exposed/ExposedContentAPI.kt

@@ -64,6 +64,11 @@ private class ContentAPIDatabaseTable(
         Json.plain.parse(Content.serializer(), get(dataColumn))
     )
 
+    override suspend fun getContentsIds(): Set<ContentId> {
+        return transaction {
+            selectAll().map { it[idColumn] }
+        }.toSet()
+    }
     override suspend fun getContentById(id: ContentId): RegisteredContent? {
         return transaction {
             select { idColumn.eq(id) }.firstOrNull() ?.asRegisteredContent()