Document supported LightRAG storage implementations in config

[Copilot]

Storage implementation types in StorageConfig had unclear descriptions. Users had no visibility into which LightRAG storage backends are actually supported.

Changes

• src/config.py: Added complete supported implementation lists to each storage field description

  • kv_storage: JsonKVStorage, RedisKVStorage, PGKVStorage, MongoKVStorage
  • vector_storage: NanoVectorDBStorage, MilvusVectorDBStorage, PGVectorStorage, FaissVectorDBStorage, QdrantVectorDBStorage, MongoVectorDBStorage
  • graph_storage: NetworkXStorage, Neo4JStorage, PGGraphStorage, MongoGraphStorage, MemgraphStorage
  • doc_status_storage: JsonDocStatusStorage, RedisDocStatusStorage, PGDocStatusStorage, MongoDocStatusStorage

• src/config.py: Fixed vector_storage default from QdrantStorage → QdrantVectorDBStorage (correct LightRAG class name)

• env.example: Updated storage section comments with complete support lists and LightRAG repo reference

vector_storage: str = Field(
    default="QdrantVectorDBStorage",
    description=(
        "Vector Storage Type. "
        "Supported: NanoVectorDBStorage, MilvusVectorDBStorage, PGVectorStorage, "
        "FaissVectorDBStorage, QdrantVectorDBStorage, MongoVectorDBStorage"
    ),
    alias="VECTOR_STORAGE"
)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Storage implementation types hardcoded</issue_title>
<issue_description>## 问题描述
存储实现类型硬编码。

受影响的文件

• src/config.py 行 170-188

硬编码的存储类型

• kv_storage: RedisKVStorage
• vector_storage: QdrantStorage
• graph_storage: MemgraphStorage
• doc_status_storage: RedisDocStatusStorage

问题

虽然可通过环境变量覆盖，但值与 LightRAG 支持的实现绑定。

解决方案

应该在代码中提供更清晰的支持列表说明。</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Storage implementation types hardcoded #21

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