feat(ai): AI助手聊天记录服务端持久化

- 图片缓存不再二次压缩，用户预览与AI收到的图片质量一致
- 新增 ai_assistant_sessions 表及 AiAssistantSession 模型
- 新增会话 API：session/list、session/save、session/delete
- 前端会话存储从 IndexedDB 切换为服务端 API，图片落盘到 uploads/assistant/{YYYYMM}/{user_id}/
- saveSessionStore 添加防抖，删除会话时同步清理内存缓存

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

database/migrations/2026_04_17_000001_create_ai_assistant_sessions_table.php

@@ -0,0 +1,33 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+class CreateAiAssistantSessionsTable extends Migration
+{
+    public function up()
+    {
+        if (Schema::hasTable('ai_assistant_sessions')) {
+            return;
+        }
+        Schema::create('ai_assistant_sessions', function (Blueprint $table) {
+            $table->bigIncrements('id');
+            $table->bigInteger('userid')->default(0)->comment('用户ID');
+            $table->string('session_key', 100)->default('')->comment('场景分类key');
+            $table->string('session_id', 100)->default('')->comment('前端生成的会话ID');
+            $table->string('scene_key', 200)->default('')->comment('具体场景标识');
+            $table->string('title', 255)->default('')->comment('会话标题');
+            $table->longText('data')->nullable()->comment('responses JSON');
+            $table->longText('images')->nullable()->comment('图片映射 {imageId: relativePath}');
+            $table->timestamps();
+            $table->index('userid', 'idx_userid');
+            $table->unique(['userid', 'session_key', 'session_id'], 'uk_user_session');
+        });
+    }
+
+    public function down()
+    {
+        Schema::dropIfExists('ai_assistant_sessions');
+    }
+}