Remove hardcoded EMBEDDING_MODEL default value

[Copilot]

EMBEDDING_MODEL had a hardcoded default in src/config.py that duplicated configuration already defined in env.example and docker-compose.yml.

Changes

• Changed EmbeddingConfig.model from optional with default to required field
• Now consistent with api_key and base_url handling in the same class

# Before
model: str = Field(
    default="Qwen/Qwen3-Embedding-0.6B",
    description="Embedding Model Name"
)

# After
model: str = Field(..., description="Embedding Model Name")

Compatibility

Docker compose files already provide fallback defaults via ${EMBEDDING_MODEL:-Qwen/Qwen3-Embedding-0.6B}, so deployment behavior is unchanged.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Embedding model default hardcoded - should use env variable</issue_title>
<issue_description>## 问题描述
Embedding 模型默认值硬编码。

受影响的文件

• src/config.py 行 47

硬编码值

• default="Qwen/Qwen3-Embedding-0.6B"

问题

虽然有 EMBEDDING_MODEL 环境变量，但代码中也硬编码。

解决方案

应该移除硬编码的默认值或保持一致。</issue_description>

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

• Fixes Embedding model default hardcoded - should use env variable #18

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