refactor(api): TOML 配置 SSOT、统一错误契约、Auth/事务加固与可观测性 (#33)

配置 SSOT（TOML + .env）
统一错误契约
Auth 与事务边界
Redis / Celery 可靠性:业务 Redis（DB/0）与 Celery broker/backend（DB/1）显式拆分；连接池、sync client
可观测性（OpenTelemetry + LGTM）

api/app/tasks/chapter_compose_tasks.py

@@ -10,18 +10,20 @@ from app.core.chapter_pipeline_lock import (
     release_chapter_pipeline_lock,
 )
 from app.core.config import settings
-from app.core.db import get_sync_db
+from app.core.db import get_sync_db, transactional_sync
 from app.core.logging import get_logger
 from app.core.memoir_pipeline_progress import merge_fanout_item
 from app.core.memoir_pipeline_trace import new_memoir_correlation_id
 from app.core.memory_compaction_schedule import schedule_memory_compaction_run
 from app.features.memoir import repo as memoir_repo
 from app.features.memoir.models import Chapter
+from app.features.memoir.constants import memoir
+from app.features.story.constants import story
 
 logger = get_logger(__name__)
 
 
-@shared_task(bind=True, max_retries=8, default_retry_delay=30)
+@shared_task(bind=True, max_retries=8, default_retry_delay=30, ignore_result=True)
 def recompose_chapter(
     self, chapter_id: str, memoir_correlation_id: str | None = None
 ) -> dict:
@@ -29,7 +31,7 @@ def recompose_chapter(
     按章节物化 canonical_markdown：仅当 markdown_compose_dirty 为 True 时执行；
     与 pipeline 共用章节级 Redis 锁，拿不到锁则跳过（依赖后续触发重试）。
     """
-    lock_ttl = int(settings.chapter_pipeline_lock_ttl_seconds)
+    lock_ttl = int(story.chapter_pipeline_lock_ttl_seconds)
     tid = str(self.request.id)
     t0 = time.perf_counter()
     merge_fanout_item(
@@ -90,10 +92,10 @@ def recompose_chapter(
                 chapter_id,
                 uid,
                 stage,
-                settings.memoir_recompose_retry_on_lock_contention,
+                memoir.recompose_retry_on_lock_contention,
                 ms,
             )
-            if settings.memoir_recompose_retry_on_lock_contention:
+            if memoir.recompose_retry_on_lock_contention:
                 countdown = max(15, min(120, lock_ttl // 4))
                 raise self.retry(countdown=countdown)
             merge_fanout_item(
@@ -106,13 +108,12 @@ def recompose_chapter(
             )
             return {"status": "skip_lock_contention"}
         try:
-            composed = memoir_repo.compose_chapter_from_story_links_sync(
-                session, chapter_id
-            )
-            session.commit()
+            with transactional_sync(session):
+                composed = memoir_repo.compose_chapter_from_story_links_sync(
+                    session, chapter_id
+                )
             user_id = uid
         except Exception as exc:
-            session.rollback()
             logger.warning(
                 "recompose_chapter failed chapter_id={} err={}", chapter_id, exc
             )