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

.env

@@ -0,0 +1,10 @@
+# Required — fill in before running
+ZHIPUAI_API_KEY=your-zhipuai-api-key-here
+STORAGE_ACCESS_KEY=your-storage-access-key
+STORAGE_SECRET_KEY=your-storage-secret-key
+STORAGE_ENDPOINT=http://rustfs:9000
+
+# Optional overrides
+BACKEND_CALLBACK_URL=http://label-backend:8080/api/ai/callback
+LOG_LEVEL=INFO
+# MAX_VIDEO_SIZE_MB=200