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_app_config_loader.py

@@ -0,0 +1,33 @@
+"""TOML configuration loader tests."""
+
+from pathlib import Path
+
+import pytest
+
+from app.core.app_config_loader import load_app_config
+
+
+FIXTURES = Path(__file__).resolve().parent / "fixtures" / "config"
+
+
+def test_load_default_only() -> None:
+    cfg = load_app_config("development", config_dir=FIXTURES / "minimal")
+    assert cfg.chat.interview_persona == "default"
+    assert cfg.deploy.enable_tts is True
+
+
+def test_staging_overlay_merges_chat_section() -> None:
+    cfg = load_app_config("staging", config_dir=FIXTURES / "merge")
+    assert cfg.chat.interview_persona == "staging_persona"
+    assert cfg.chat.interview_temperature == 0.93
+    assert cfg.deploy.mock_sms_login_enabled is True
+
+
+def test_unknown_top_level_key_rejected(tmp_path: Path) -> None:
+    bad = tmp_path / "default.toml"
+    bad.write_text(
+        '[deploy]\nenable_tts = true\n\n[typo_section]\nfoo = 1\n',
+        encoding="utf-8",
+    )
+    with pytest.raises(Exception):
+        load_app_config("development", config_dir=tmp_path)