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/ww_api/services/project_deps.py

@@ -20,6 +20,7 @@ from ww_core.domain import ForeshadowLedgerRepo, SqlForeshadowLedgerRepo
 from ww_core.domain.chapter_repo import ChapterRepo, SqlChapterRepo
 from ww_core.domain.character_repo import CharacterWriteRepo, SqlCharacterWriteRepo
 from ww_core.domain.digest_repo import DigestAppendRepo, SqlDigestAppendRepo
+from ww_core.domain.injection_repo import InjectionOverrideRepo, SqlInjectionOverrideRepo
 from ww_core.domain.job_repo import JobRepo, SqlJobRepo
 from ww_core.domain.outline_write_repo import OutlineWriteRepo, SqlOutlineWriteRepo
 from ww_core.domain.project_repo import ProjectRepo, SqlProjectRepo
@@ -102,6 +103,16 @@ def get_memory_repos(
     return sql_memory_repos(session)
 
 
+def get_injection_repo(
+    session: Annotated[AsyncSession, Depends(get_session)],
+) -> InjectionOverrideRepo:
+    """本章注入覆盖读/写 repo（B0 可控版 GET/PUT injection + draft 同读）。
+
+    upsert 只 flush，端点提交（与其它写侧一致）。测试经 `app.dependency_overrides` 注 fake。
+    """
+    return SqlInjectionOverrideRepo(session)
+
+
 def get_review_repo(
     session: Annotated[AsyncSession, Depends(get_session)],
 ) -> ReviewRepo: