feat: 创建 LobsterDiary 模型，支持数据库存储日记

- 创建 LobsterDiary 模型
  * 关联龙虾（ForeignKey）
  * 日期、标题、内容
  * 分类（成才之路/工作记忆/技术笔记）
  * 标签（JSONField）
  * Embedding 字段（预留 RAG 支持）
  * 数据库索引优化

- 数据库迁移
  * 添加 LobsterDiary 表
  * 添加索引：lobster+date, category+date, date

- 导入脚本
  * 创建 import_diaries 管理命令
  * 导入飞行侠的成才之路日记（3 篇）

- 更新 API
  * /api/lobsters/<id>/diary/dates/ - 从数据库查询
  * /api/lobsters/<id>/diary/<date>/ - 从数据库读取

- PostgreSQL 配置模板
  * settings_postgresql.py
  * 准备好 PostgreSQL 迁移

技术栈：SQLite（当前） → PostgreSQL（未来）
RAG 支持：预留 embedding 字段，未来可扩展

🗄️ 日记正式进入数据库时代！

code/backend/lobsters/migrations/0002_lobster_workspace_lobsterdiary_and_more.py

@@ -0,0 +1,109 @@
+# Generated by Django 4.2 on 2026-04-03 09:37
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("lobsters", "0001_initial"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="lobster",
+            name="workspace",
+            field=models.CharField(
+                blank=True, default="", max_length=100, verbose_name="工作区"
+            ),
+        ),
+        migrations.CreateModel(
+            name="LobsterDiary",
+            fields=[
+                (
+                    "id",
+                    models.BigAutoField(
+                        auto_created=True,
+                        primary_key=True,
+                        serialize=False,
+                        verbose_name="ID",
+                    ),
+                ),
+                ("date", models.DateField(verbose_name="日期")),
+                ("title", models.CharField(max_length=200, verbose_name="标题")),
+                ("content", models.TextField(verbose_name="内容")),
+                (
+                    "category",
+                    models.CharField(
+                        choices=[
+                            ("chengcai", "成才之路"),
+                            ("memory", "工作记忆"),
+                            ("tech", "技术笔记"),
+                            ("other", "其他"),
+                        ],
+                        default="other",
+                        max_length=50,
+                        verbose_name="分类",
+                    ),
+                ),
+                (
+                    "tags",
+                    models.JSONField(blank=True, default=list, verbose_name="标签"),
+                ),
+                (
+                    "embedding",
+                    models.TextField(
+                        blank=True, null=True, verbose_name="文本向量 (JSON 格式)"
+                    ),
+                ),
+                (
+                    "embedding_model",
+                    models.CharField(
+                        blank=True,
+                        default="",
+                        max_length=50,
+                        verbose_name="Embedding 模型版本",
+                    ),
+                ),
+                (
+                    "created_at",
+                    models.DateTimeField(auto_now_add=True, verbose_name="创建时间"),
+                ),
+                (
+                    "updated_at",
+                    models.DateTimeField(auto_now=True, verbose_name="更新时间"),
+                ),
+                (
+                    "lobster",
+                    models.ForeignKey(
+                        on_delete=django.db.models.deletion.CASCADE,
+                        related_name="diaries",
+                        to="lobsters.lobster",
+                        verbose_name="龙虾",
+                    ),
+                ),
+            ],
+            options={
+                "verbose_name": "龙虾日记",
+                "verbose_name_plural": "龙虾日记",
+                "ordering": ["-date", "-created_at"],
+            },
+        ),
+        migrations.AddIndex(
+            model_name="lobsterdiary",
+            index=models.Index(
+                fields=["lobster", "date"], name="lobsters_lo_lobster_5895f8_idx"
+            ),
+        ),
+        migrations.AddIndex(
+            model_name="lobsterdiary",
+            index=models.Index(
+                fields=["category", "date"], name="lobsters_lo_categor_177677_idx"
+            ),
+        ),
+        migrations.AddIndex(
+            model_name="lobsterdiary",
+            index=models.Index(fields=["date"], name="lobsters_lo_date_d11f64_idx"),
+        ),
+    ]