feat(b0): 注入透明可控版后端 — PUT override(pin/排除/recent_n) + 选择函数加参 + draft 同读

- selection.select_relevant_entities 加 pinned/excluded frozenset 入参；新理由 author_pin（末位）
- assemble 加 override 关键字参（recent_n 覆盖回看章数 + pin/排除）；GET/PUT/draft 同读同一覆盖（不变量 #6 作者兜底）
- 新 domain/injection_repo.py（InjectionOverride/EntityRef/SqlInjectionOverrideRepo，upsert 只 flush）
- 新表 chapter_injection（迁移 ad2c4c663daf，唯一 project_id+chapter_no；复用 outline 行已否决）
- PUT/GET injection 端点 + InjectionOverrideRequest/回显 pinned/excluded；recent_n 1..20 校验 422
- 单测：selection pin/排除/exclude>pin + assemble override + 端点 PUT/404/422；regen TS 客户端
- 门禁绿：ruff/format · mypy 163 · alembic 无漂移 · pytest 476

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/db/migrations/versions/ad2c4c663daf_t_b0_chapter_injection_override_table.py

@@ -0,0 +1,58 @@
+"""T_B0 chapter_injection override table
+
+Revision ID: ad2c4c663daf
+Revises: 1f011c42bd4d
+Create Date: 2026-06-20 11:51:00.494169
+"""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+import sqlalchemy as sa
+from alembic import op
+from sqlalchemy.dialects import postgresql
+
+revision: str = "ad2c4c663daf"
+down_revision: str | None = "1f011c42bd4d"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.create_table(
+        "chapter_injection",
+        sa.Column("project_id", sa.Uuid(), nullable=False),
+        sa.Column("chapter_no", sa.Integer(), nullable=False),
+        sa.Column(
+            "pinned",
+            postgresql.JSONB(astext_type=sa.Text()),
+            server_default=sa.text("'[]'"),
+            nullable=False,
+        ),
+        sa.Column(
+            "excluded",
+            postgresql.JSONB(astext_type=sa.Text()),
+            server_default=sa.text("'[]'"),
+            nullable=False,
+        ),
+        sa.Column("recent_n", sa.Integer(), nullable=True),
+        sa.Column("id", sa.Uuid(), server_default=sa.text("gen_random_uuid()"), nullable=False),
+        sa.Column("updated_at", sa.DateTime(), server_default=sa.text("now()"), nullable=False),
+        sa.Column("created_at", sa.DateTime(), server_default=sa.text("now()"), nullable=False),
+        sa.ForeignKeyConstraint(["project_id"], ["projects.id"], ondelete="CASCADE"),
+        sa.PrimaryKeyConstraint("id"),
+        sa.UniqueConstraint("project_id", "chapter_no"),
+    )
+    op.create_index(
+        op.f("ix_chapter_injection_project_id"), "chapter_injection", ["project_id"], unique=False
+    )
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_index(op.f("ix_chapter_injection_project_id"), table_name="chapter_injection")
+    op.drop_table("chapter_injection")
+    # ### end Alembic commands ###