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_memory_enrichment_baseline.py

@@ -10,6 +10,7 @@ from app.features.memory.enrichment import enrich_memory_after_ingest_async
 from app.features.memory.llm_schemas import EnrichmentPayload
 from app.features.memory.models import MemorySource
 from app.features.user.models import User
+from app.features.memory.constants import memory
 
 
 def test_enrichment_payload_roundtrip() -> None:
@@ -31,7 +32,7 @@ async def test_enrich_memory_after_ingest_async_single_llm_call(
 ) -> None:
     from app.features.memory import enrichment as mod
 
-    monkeypatch.setattr("app.core.config.settings.memory_enrichment_enabled", True)
+    monkeypatch.setattr("app.features.memory.constants.memory.enrichment_enabled", True)
 
     invoke_count = {"n": 0}
 
@@ -113,7 +114,7 @@ async def test_enrich_memory_skips_when_parse_returns_none(
 ) -> None:
     from app.features.memory import enrichment as mod
 
-    monkeypatch.setattr("app.core.config.settings.memory_enrichment_enabled", True)
+    monkeypatch.setattr("app.features.memory.constants.memory.enrichment_enabled", True)
 
     async def fake_run(*_args, **_kwargs):
         return None