feat(backend): persist tasks and generated message history

- add EntityTask domain and CRUD API backed by PostgreSQL

- relate generated messages directly to tasks and delete on task removal

- move preview generation to backend Llama endpoint

- migrate frontend task APIs from localStorage to backend endpoints

- update tests and CLAUDE rules for backend-owned LLM/persistence

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

@@ -0,0 +1,11 @@
+package com.condado.newsletter.repository
+
+import com.condado.newsletter.model.EntityTask
+import com.condado.newsletter.model.VirtualEntity
+import org.springframework.data.jpa.repository.JpaRepository
+import java.util.UUID
+
+interface EntityTaskRepository : JpaRepository<EntityTask, UUID> {
+    fun findAllByActiveTrue(): List<EntityTask>
+    fun findAllByVirtualEntity(virtualEntity: VirtualEntity): List<EntityTask>
+}

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

@@ -0,0 +1,12 @@
+package com.condado.newsletter.repository
+
+import com.condado.newsletter.model.GeneratedMessageHistory
+import org.springframework.data.jpa.repository.JpaRepository
+import java.util.UUID
+
+interface GeneratedMessageHistoryRepository : JpaRepository<GeneratedMessageHistory, UUID> {
+    fun findAllByTask_IdOrderByCreatedAtDesc(taskId: UUID): List<GeneratedMessageHistory>
+    fun countByTask_Id(taskId: UUID): Long
+    fun deleteByIdAndTask_Id(id: UUID, taskId: UUID): Long
+    fun deleteAllByTask_Id(taskId: UUID)
+}