feat: make Tenant ID length constraints configurable via environment variables

[Copilot]

Tenant ID length limits (min 3, max 50) were hardcoded in src/tenant_deps.py, limiting business flexibility for different use cases.

Changes

• src/config.py: Add tenant_id_min_length and tenant_id_max_length fields to MultiTenantConfig
• src/tenant_deps.py: Use config values instead of hardcoded constants for validation and FastAPI Query params
• env.example: Document new environment variables

Usage

# Custom length constraints (defaults: 3-50)
TENANT_ID_MIN_LENGTH=5
TENANT_ID_MAX_LENGTH=100

Config values are cached at module load time—requires service restart to take effect.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Tenant ID length constraints hardcoded - reduces business flexibility</issue_title>
<issue_description>## 问题描述
Tenant ID 长度限制硬编码，降低业务灵活性。

受影响的文件

• src/tenant_deps.py 行 36-37

硬编码值

• 最小 3 字符
• 最大 50 字符

问题

某些业务场景可能需要不同的限制。

解决方案

应该配置以下环境变量：

• TENANT_ID_MIN_LENGTH
• TENANT_ID_MAX_LENGTH</issue_description>

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

• Fixes Tenant ID length constraints hardcoded - reduces business flexibility #16

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.