feat(backend): implement step 3 — VirtualEntityRepository and DispatchLogRepository

backend/src/main/kotlin/com/condado/newsletter/repository/DispatchLogRepository.kt

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+package com.condado.newsletter.repository
+
+import com.condado.newsletter.model.DispatchLog
+import com.condado.newsletter.model.VirtualEntity
+import org.springframework.data.jpa.repository.JpaRepository
+import java.util.Optional
+import java.util.UUID
+
+/**
+ * Repository for [DispatchLog] with custom query methods.
+ */
+interface DispatchLogRepository : JpaRepository<DispatchLog, UUID> {
+
+    /** Returns all dispatch logs for the given [VirtualEntity]. */
+    fun findAllByVirtualEntity(virtualEntity: VirtualEntity): List<DispatchLog>
+
+    /** Returns the most recent dispatch log for the given [VirtualEntity], or empty if none exist. */
+    fun findTopByVirtualEntityOrderByDispatchedAtDesc(virtualEntity: VirtualEntity): Optional<DispatchLog>
+}

backend/src/main/kotlin/com/condado/newsletter/repository/VirtualEntityRepository.kt

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+package com.condado.newsletter.repository
+
+import com.condado.newsletter.model.VirtualEntity
+import org.springframework.data.jpa.repository.JpaRepository
+import java.util.Optional
+import java.util.UUID
+
+/**
+ * Repository for [VirtualEntity] with custom query methods.
+ */
+interface VirtualEntityRepository : JpaRepository<VirtualEntity, UUID> {
+
+    /** Returns all entities where [VirtualEntity.active] is true. */
+    fun findAllByActiveTrue(): List<VirtualEntity>
+
+    /** Finds an entity by its unique email address. */
+    fun findByEmail(email: String): Optional<VirtualEntity>
+}