支持 LDAP 登录与认证模式配置

[lshy0000]

Description

Add configurable LDAP/AD authentication support.

• Add LDAP/AD login flow with configurable user lookup and bind.
• Add LDAP_LOGIN_MODE=optional|required to support mixed login or LDAP-only login.
• Add AUTH_REGISTRATION_MODE=self_serve|invite_only to control whether self-registration is available.
• Update login page labels to support username or email login.
• Add config parsing tests for authentication-related environment variables.

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 💥 Breaking change
• 📚 Documentation update
• 🎨 Refactor
• ⚡ Performance improvement
• 🧪 Test
• 🔧 Configuration / Build / CI

Related Issue

N/A

Testing

Not run locally because the current environment does not have the Go toolchain installed.

Checklist

• make fmt && make lint && make test pass locally
• Self-reviewed the code
• Added/updated tests covering the change
• Updated related documentation (README, docs/, Swagger annotations, etc.)
• Breaking changes are clearly called out in the description above

Screenshots / Recordings

N/A