Create server configuration system

[inureyes]

Summary

Create a comprehensive configuration system for bssh-server that supports YAML configuration files, environment variables, and CLI argument overrides.

Parent Epic

• Implement bssh-server with SFTP/SCP support #123 - bssh-server 추가 구현
• Depends on: Create shared module structure for client/server code reuse #124 (shared module structure)

Design Goals

1. Container-friendly: CLI args can override all config options
2. Hierarchical: CLI > Environment > Config File > Defaults
3. Validated: Configuration errors caught at startup
4. Documented: All options documented in code and generated docs

Implementation Status

All core functionality has been implemented:

✅ Configuration types defined with serde support
✅ YAML configuration file loading with default search paths
✅ Environment variable overrides (BSSH_* prefix)
✅ Configuration validation at startup
✅ Default value functions for all optional fields
✅ Config template generation function
✅ Comprehensive documentation on all config fields
✅ Full test coverage (25 tests passing)
✅ Backward compatibility maintained with existing ServerConfig API

Files Created/Modified

File	Status	Description
src/server/config/mod.rs	✅ Created	Module exports and backward compatibility layer
src/server/config/types.rs	✅ Created	Configuration types with serde support
src/server/config/loader.rs	✅ Created	Config loader with env/validation
Cargo.toml	✅ Modified	Added ipnetwork = "0.20"

Acceptance Criteria

• All configuration types defined with serde
• YAML configuration file loading
• Environment variable overrides
• CLI argument overrides (preparation for Create server CLI interface (bssh-server binary) #131)
• Configuration validation at startup
• Default value functions for all optional fields
• Config template generation command
• Documentation comments on all config fields
• Tests passing