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/tests/test_chapter_cover_enqueue_redis.py

@@ -0,0 +1,76 @@
+import pytest
+
+from app.tasks import chapter_cover_enqueue as enqueue_mod
+
+
+def test_chapter_cover_enqueue_reuses_sync_redis_client(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    class FakeRedis:
+        def __init__(self) -> None:
+            self.set_calls = 0
+
+        def set(self, *args, **kwargs):
+            self.set_calls += 1
+            return True
+
+        def delete(self, key: str) -> None:
+            return None
+
+    shared_client = FakeRedis()
+
+    def fake_get_sync_redis(*, decode_responses: bool):
+        assert decode_responses is True
+        return shared_client
+
+    class Chapter:
+        category = "childhood"
+        cover_asset_id = None
+        story_links = [object()]
+        images = []
+
+    class FakeCoverTask:
+        def delay(self, chapter_id: str) -> None:
+            return None
+
+    monkeypatch.setattr(enqueue_mod, "get_sync_redis", fake_get_sync_redis)
+    monkeypatch.setattr(
+        enqueue_mod,
+        "_load_chapter_for_enqueue_sync",
+        lambda chapter_id: Chapter(),
+    )
+    monkeypatch.setattr(
+        enqueue_mod,
+        "chapter_has_story_links",
+        lambda chapter: True,
+    )
+    monkeypatch.setattr(
+        enqueue_mod,
+        "effective_chapter_markdown_for_cover_gates",
+        lambda chapter: "body",
+    )
+    monkeypatch.setattr(
+        enqueue_mod,
+        "strip_image_placeholders",
+        lambda text: text,
+    )
+    monkeypatch.setattr(
+        enqueue_mod,
+        "chapter_eligible_for_cover_by_inline_body_image_count",
+        lambda chapter, markdown: True,
+    )
+    monkeypatch.setattr(
+        enqueue_mod,
+        "primary_chapter_memoir_image",
+        lambda chapter: None,
+    )
+    import app.tasks.chapter_cover_tasks as cover_tasks
+
+    monkeypatch.setattr(cover_tasks, "generate_chapter_cover", FakeCoverTask())
+
+    ok = enqueue_mod.try_enqueue_generate_chapter_cover("chapter-1", source="http")
+    assert ok is True
+    assert shared_client.set_calls == 1
+
+    enqueue_mod.try_enqueue_generate_chapter_cover("chapter-2", source="http")
+    assert shared_client.set_calls == 2