feat: add config loading module with YAML support

实现应用配置加载模块，支持从 YAML 文件或默认值加载配置。
覆盖 ChromaDB、嵌入模型、文档分块和 HTTP 服务配置。

- src/core/config.py: 配置数据类 (AppConfig/ChromaConfig/EmbedConfig/ChunkConfig/ServerConfig) + load_config 函数
- tests/test_config.py: 5 个单元测试覆盖默认值、API 模式和文件加载

Co-Authored-By: Claude <noreply@anthropic.com>

tests/test_config.py

@@ -0,0 +1,82 @@
+"""配置加载模块测试."""
+import tempfile
+from pathlib import Path
+
+import pytest
+
+from src.core.config import AppConfig, EmbedConfig, ChunkConfig, load_config
+
+
+class TestEmbedConfig:
+    """嵌入配置解析."""
+
+    def test_local_mode_defaults(self):
+        """默认 local 模式，带默认模型名."""
+        data = {"embed": {"mode": "local"}}
+        cfg = AppConfig(**data)
+        assert cfg.embed.mode == "local"
+        assert cfg.embed.local_model == "BAAI/bge-small-zh-v1.5"
+        assert cfg.embed.api_base == ""
+
+    def test_api_mode_fields(self):
+        """api 模式下 api_base 和 api_key 可设置."""
+        data = {
+            "embed": {
+                "mode": "api",
+                "api_base": "https://api.openai.com/v1",
+                "api_key": "sk-test",
+            }
+        }
+        cfg = AppConfig(**data)
+        assert cfg.embed.mode == "api"
+        assert cfg.embed.api_base == "https://api.openai.com/v1"
+        assert cfg.embed.api_key == "sk-test"
+
+
+class TestChunkConfig:
+    """分块配置解析."""
+
+    def test_default_values(self):
+        """分块默认值正确."""
+        data = {"chunk": {}}
+        cfg = AppConfig(**data)
+        assert cfg.chunk.max_size == 1000
+        assert cfg.chunk.overlap == 100
+
+
+class TestLoadConfig:
+    """load_config 函数测试."""
+
+    def test_load_from_yaml_file(self):
+        """从 YAML 文件加载配置."""
+        yaml_content = """
+chroma:
+  persist_dir: /tmp/test_data
+  collection_name: test_collection
+embed:
+  mode: local
+chunk:
+  max_size: 500
+server:
+  port: 9000
+"""
+        with tempfile.NamedTemporaryFile(
+            mode="w", suffix=".yaml", delete=False
+        ) as f:
+            f.write(yaml_content)
+            tmp_path = f.name
+
+        try:
+            cfg = load_config(tmp_path)
+            assert cfg.chroma.persist_dir == "/tmp/test_data"
+            assert cfg.chroma.collection_name == "test_collection"
+            assert cfg.chunk.max_size == 500
+            assert cfg.server.port == 9000
+        finally:
+            Path(tmp_path).unlink()
+
+    def test_load_missing_file_uses_defaults(self):
+        """配置文件不存在时使用默认值."""
+        cfg = load_config("/nonexistent/config.yaml")
+        assert cfg.chroma.persist_dir == "./data"
+        assert cfg.embed.mode == "local"