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/dto/EntityTaskDtos.kt

@@ -0,0 +1,48 @@
+package com.condado.newsletter.dto
+
+import com.condado.newsletter.model.EntityTask
+import jakarta.validation.constraints.NotBlank
+import jakarta.validation.constraints.NotNull
+import java.time.LocalDateTime
+import java.util.UUID
+
+data class EntityTaskCreateDto(
+    @field:NotNull val entityId: UUID,
+    @field:NotBlank val name: String,
+    @field:NotBlank val prompt: String,
+    @field:NotBlank val scheduleCron: String,
+    @field:NotBlank val emailLookback: String
+)
+
+data class EntityTaskUpdateDto(
+    @field:NotNull val entityId: UUID,
+    @field:NotBlank val name: String,
+    @field:NotBlank val prompt: String,
+    @field:NotBlank val scheduleCron: String,
+    @field:NotBlank val emailLookback: String
+)
+
+data class EntityTaskResponseDto(
+    val id: UUID,
+    val entityId: UUID,
+    val name: String,
+    val prompt: String,
+    val scheduleCron: String,
+    val emailLookback: String,
+    val active: Boolean,
+    val createdAt: LocalDateTime?
+) {
+    companion object {
+        fun from(task: EntityTask): EntityTaskResponseDto =
+            EntityTaskResponseDto(
+                id = task.id!!,
+                entityId = task.virtualEntity.id!!,
+                name = task.name,
+                prompt = task.prompt,
+                scheduleCron = task.scheduleCron,
+                emailLookback = task.emailLookback,
+                active = task.active,
+                createdAt = task.createdAt
+            )
+    }
+}