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>

apps/api/tests/test_projects.py

@@ -67,6 +67,13 @@ class _StubProjectSpecRepo:
         return ProjectSpecView(title="测试作品", premise="测试前提")
 
 
+class _NoInjectionRepo:
+    """空注入覆盖 stub：draft 端点读它得 None（纯自动选择），不打真 DB。"""
+
+    async def get(self, project_id: uuid.UUID, chapter_no: int) -> None:
+        return None
+
+
 def _empty_memory_repos() -> MemoryRepos:
     return MemoryRepos(
         outline=_EmptyOutlineRepo(),
@@ -92,6 +99,7 @@ def _make_client(
     from ww_api.main import create_app
     from ww_api.services.project_deps import (
         get_chapter_repo,
+        get_injection_repo,
         get_memory_repos,
         get_project_repo,
         get_writer_gateway,
@@ -106,6 +114,8 @@ def _make_client(
     app.dependency_overrides[get_chapter_repo] = lambda: chapter_repo
     app.dependency_overrides[get_memory_repos] = _empty_memory_repos
     app.dependency_overrides[get_writer_gateway] = lambda: gateway
+    # draft 端点现读注入覆盖：注空 stub，避免单测打真 DB（无覆盖 → 纯自动选择）。
+    app.dependency_overrides[get_injection_repo] = lambda: _NoInjectionRepo()
     transport = httpx.ASGITransport(app=app)
     client = httpx.AsyncClient(transport=transport, base_url="http://test")
     return client, project_repo, chapter_repo, gateway