feat: Phase 1+2 — project setup and core infrastructure

- requirements.txt, config.yaml, .env, Dockerfile, docker-compose.yml
- app/core: config (YAML+env override), logging (JSON structured),
  exceptions (typed hierarchy), json_utils (Markdown fence stripping)
- app/clients: LLMClient ABC + ZhipuAIClient (run_in_executor),
  StorageClient ABC + RustFSClient (boto3 head_object for size check)
- app/main.py: FastAPI app with health endpoint and router registration
- app/core/dependencies.py: lru_cache singleton factories
- tests/conftest.py: mock_llm, mock_storage, test_app, client fixtures
- pytest.ini: asyncio_mode=auto
- 11 unit tests passing

app/core/config.py

@@ -0,0 +1,46 @@
+import os
+from functools import lru_cache
+from pathlib import Path
+from typing import Any
+
+import yaml
+from dotenv import load_dotenv
+
+load_dotenv()
+
+# Maps environment variable names to nested YAML key paths
+_ENV_OVERRIDES: dict[str, list[str]] = {
+    "ZHIPUAI_API_KEY": ["zhipuai", "api_key"],
+    "STORAGE_ACCESS_KEY": ["storage", "access_key"],
+    "STORAGE_SECRET_KEY": ["storage", "secret_key"],
+    "STORAGE_ENDPOINT": ["storage", "endpoint"],
+    "BACKEND_CALLBACK_URL": ["backend", "callback_url"],
+    "LOG_LEVEL": ["server", "log_level"],
+    "MAX_VIDEO_SIZE_MB": ["video", "max_file_size_mb"],
+}
+
+_CONFIG_PATH = Path(__file__).parent.parent.parent / "config.yaml"
+
+
+def _set_nested(cfg: dict, keys: list[str], value: Any) -> None:
+    for key in keys[:-1]:
+        cfg = cfg.setdefault(key, {})
+    # Coerce numeric env vars
+    try:
+        value = int(value)
+    except (TypeError, ValueError):
+        pass
+    cfg[keys[-1]] = value
+
+
+@lru_cache(maxsize=1)
+def get_config() -> dict:
+    with open(_CONFIG_PATH, "r", encoding="utf-8") as f:
+        cfg: dict = yaml.safe_load(f)
+
+    for env_var, key_path in _ENV_OVERRIDES.items():
+        value = os.environ.get(env_var)
+        if value is not None:
+            _set_nested(cfg, key_path, value)
+
+    return cfg