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/ww_db/models.py

@@ -152,6 +152,24 @@ class Chapter(UuidPk, CreatedAt, Base):
     version: Mapped[int] = mapped_column(Integer, nullable=False, server_default=text("1"))
 
 
+class ChapterInjection(UuidPk, TimestampedMixin, Base):
+    """本章注入覆盖（B0 可控版）：作者对确定性自动选择的微调，每章一行。
+
+    与 `outline.beats` 分表存放（复用 outline 行会污染 beats/prompt，已否决）。
+    pinned/excluded 为 JSONB list[{kind,name}]；recent_n 覆盖近况回看章数（NULL=用默认）。
+    """
+
+    __tablename__ = "chapter_injection"
+    __table_args__ = (UniqueConstraint("project_id", "chapter_no"),)
+    project_id: Mapped[uuid.UUID] = mapped_column(
+        ForeignKey("projects.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    chapter_no: Mapped[int] = mapped_column(Integer, nullable=False)
+    pinned: Mapped[list[Any]] = mapped_column(JSONB, server_default=text("'[]'"))
+    excluded: Mapped[list[Any]] = mapped_column(JSONB, server_default=text("'[]'"))
+    recent_n: Mapped[int | None] = mapped_column(Integer)
+
+
 class ChapterReview(UuidPk, CreatedAt, Base):
     __tablename__ = "chapter_reviews"
     project_id: Mapped[uuid.UUID] = mapped_column(