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_alembic_migration_policy.py

@@ -39,7 +39,7 @@ def _script_dir() -> ScriptDirectory:
 
 def test_single_alembic_head() -> None:
     heads = _script_dir().get_heads()
-    assert heads == ["0019_align_legacy_schema"], f"unexpected heads: {heads}"
+    assert heads == ["0021_memory_source_segment_id"], f"unexpected heads: {heads}"
 
 
 def test_no_withdrawn_revision_ids_in_tree() -> None:
@@ -78,11 +78,11 @@ def test_all_revisions_have_unique_ids() -> None:
     assert len(ids) == len(set(ids)), "duplicate revision ids"
 
 
-def test_revision_chain_reaches_0019_from_0018() -> None:
+def test_revision_chain_reaches_0021_from_0020() -> None:
     script = _script_dir()
-    rev = script.get_revision("0019_align_legacy_schema")
+    rev = script.get_revision("0021_memory_source_segment_id")
     assert rev is not None
-    assert rev.down_revision == "0018_users_language_preference"
+    assert rev.down_revision == "0020_refresh_rt_lineage"
 
 
 def test_no_autogenerate_introspection_backfill_pattern() -> None: