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/test/kotlin/com/condado/newsletter/service/TaskGeneratedMessageServiceTest.kt

@@ -0,0 +1,84 @@
+package com.condado.newsletter.service
+
+import com.condado.newsletter.dto.TaskPreviewEntityDto
+import com.condado.newsletter.dto.TaskPreviewGenerateRequestDto
+import com.condado.newsletter.dto.TaskPreviewTaskDto
+import com.condado.newsletter.model.EntityTask
+import com.condado.newsletter.model.GeneratedMessageHistory
+import com.condado.newsletter.model.VirtualEntity
+import com.condado.newsletter.repository.EntityTaskRepository
+import com.condado.newsletter.repository.GeneratedMessageHistoryRepository
+import io.mockk.every
+import io.mockk.mockk
+import io.mockk.slot
+import io.mockk.verify
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.api.Test
+import java.util.UUID
+
+class TaskGeneratedMessageServiceTest {
+
+    private val generatedMessageHistoryRepository: GeneratedMessageHistoryRepository = mockk()
+    private val entityTaskRepository: EntityTaskRepository = mockk()
+    private val llamaPreviewService: LlamaPreviewService = mockk()
+
+    private val service = TaskGeneratedMessageService(
+        generatedMessageHistoryRepository = generatedMessageHistoryRepository,
+        entityTaskRepository = entityTaskRepository,
+        llamaPreviewService = llamaPreviewService
+    )
+
+    @Test
+    fun should_generateAndPersistMessage_when_generateAndSaveCalled() {
+        val taskId = UUID.randomUUID()
+        val entity = VirtualEntity(name = "Entity", email = "e@x.com", jobTitle = "Ops").apply { id = UUID.randomUUID() }
+        val task = EntityTask(
+            virtualEntity = entity,
+            name = "Task",
+            prompt = "Prompt",
+            scheduleCron = "0 9 * * 1",
+            emailLookback = "last_week"
+        ).apply { id = taskId }
+        val captured = slot<GeneratedMessageHistory>()
+
+        every { llamaPreviewService.generate(any()) } returns "SUBJECT: Test\nBODY:\nGenerated by backend"
+        every { entityTaskRepository.findById(taskId) } returns java.util.Optional.of(task)
+        every { generatedMessageHistoryRepository.countByTask_Id(taskId) } returns 0
+        every { generatedMessageHistoryRepository.save(capture(captured)) } answers {
+            captured.captured.apply {
+                id = UUID.fromString("00000000-0000-0000-0000-000000000001")
+            }
+        }
+
+        val response = service.generateAndSave(taskId, sampleRequest())
+
+        assertThat(response.id).isEqualTo(UUID.fromString("00000000-0000-0000-0000-000000000001"))
+        assertThat(response.taskId).isEqualTo(taskId)
+        assertThat(response.label).isEqualTo("Message #1")
+        assertThat(response.content).contains("Generated by backend")
+        assertThat(captured.captured.task.id).isEqualTo(taskId)
+
+        verify(exactly = 1) { llamaPreviewService.generate(any()) }
+        verify(exactly = 1) { generatedMessageHistoryRepository.save(any()) }
+    }
+
+    private fun sampleRequest() = TaskPreviewGenerateRequestDto(
+        entity = TaskPreviewEntityDto(
+            id = UUID.randomUUID().toString(),
+            name = "Entity A",
+            email = "entity@example.com",
+            jobTitle = "Ops",
+            personality = "Formal",
+            scheduleCron = "0 9 * * 1",
+            contextWindowDays = 3,
+            active = true
+        ),
+        task = TaskPreviewTaskDto(
+            entityId = UUID.randomUUID().toString(),
+            name = "Morning Blast",
+            prompt = "Talk about coffee",
+            scheduleCron = "0 8 * * 1-5",
+            emailLookback = "last_week"
+        )
+    )
+}