feat(db): ai_messages 表 + 迁移——作者↔AI 往复聊天记录 append-only 侧表（AC-1）

计划 §2 canonical DDL：AiMessage(UuidPk, CreatedAt, Base)，project_id(FK CASCADE)/
chapter_no(nullable=项目级)/thread_id/seq(repo 批内定序)/kind/tool_key/role(CHECK)/
content/meta(JSONB '{}')，append-only 无 updated_at。复合索引 (project_id,chapter_no,
created_at DESC) 手写 sa.text DESC + FK 单列索引。迁移 ceabd3c2c00d(down=f6a7b8c9d0e1)，
autogenerate 仅捕获 ai_messages 无杂散漂移。T0.2 后第一张新业务表；旁路侧记录非手稿真源，
assemble()/agent 永不读（守 #1/#3/#6）。契约 C2 扩已「稳定」。

packages/db/migrations/versions/ceabd3c2c00d_add_ai_messages_table_chat_history.py

@@ -0,0 +1,66 @@
+"""add ai_messages table (chat history)
+
+作者↔AI 往复留痕 append-only 侧表（AC-1）——非手稿真源，永不进 assemble()/agent 生成链。
+复合索引 ix_ai_messages_project_chapter 手写 created_at DESC 表达式（沿用 chapter_reviews
+证明可过 alembic check 的形态；autogenerate 无法可靠还原 DESC）。role CHECK 兜底二元枚举。
+
+Revision ID: ceabd3c2c00d
+Revises: f6a7b8c9d0e1
+Create Date: 2026-07-09 16:35:14.948502
+"""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+import sqlalchemy as sa
+from alembic import op
+from sqlalchemy.dialects import postgresql
+
+revision: str = "ceabd3c2c00d"
+down_revision: str | None = "f6a7b8c9d0e1"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "ai_messages",
+        sa.Column("project_id", sa.Uuid(), nullable=False),
+        sa.Column("chapter_no", sa.Integer(), nullable=True),
+        sa.Column("thread_id", sa.Uuid(), nullable=False),
+        sa.Column("seq", sa.Integer(), nullable=False),
+        sa.Column("kind", sa.Text(), nullable=False),
+        sa.Column("tool_key", sa.Text(), nullable=True),
+        sa.Column("role", sa.Text(), nullable=False),
+        sa.Column("content", sa.Text(), nullable=False),
+        sa.Column(
+            "meta",
+            postgresql.JSONB(astext_type=sa.Text()),
+            server_default=sa.text("'{}'"),
+            nullable=False,
+        ),
+        sa.Column("id", sa.Uuid(), server_default=sa.text("gen_random_uuid()"), nullable=False),
+        sa.Column(
+            "created_at",
+            sa.DateTime(timezone=True),
+            server_default=sa.text("now()"),
+            nullable=False,
+        ),
+        sa.CheckConstraint("role IN ('author', 'ai')", name="ck_ai_messages_role"),
+        sa.ForeignKeyConstraint(["project_id"], ["projects.id"], ondelete="CASCADE"),
+        sa.PrimaryKeyConstraint("id"),
+    )
+    op.create_index(
+        "ix_ai_messages_project_chapter",
+        "ai_messages",
+        ["project_id", "chapter_no", sa.text("created_at DESC")],
+        unique=False,
+    )
+    op.create_index(op.f("ix_ai_messages_project_id"), "ai_messages", ["project_id"], unique=False)
+
+
+def downgrade() -> None:
+    op.drop_index(op.f("ix_ai_messages_project_id"), table_name="ai_messages")
+    op.drop_index("ix_ai_messages_project_chapter", table_name="ai_messages")
+    op.drop_table("ai_messages")