Refactor: Replace monolithic config with modular ClientConfig and ServerConfig

[philippschroeppel]

Summary

Replaces the monolithic config.rs with a modular configuration system that separates client-side and server-side concerns. The modular configuration is based on #10,

Changes

• Renamed structures: GlobalConfig → ClientConfig, ServerConfig → ClientServerConfig
• Modular server config: Split Config into ServerConfig, RedisConfig, KafkaConfig, DatabaseConfig, ServerAuthConfig, TailscaleConfig
• Improved field access: CONFIG.auth_server → SERVER_CONFIG.auth.url
• Better Redis handling: Added RedisConfig.get_url() method for cleaner URL construction
• Enhanced Kafka config: Structured KafkaConfig with proper timeout parsing
• Consistent naming: SERVER_CONFIG static instance for server-side config

Breaking Changes

• GlobalConfig → ClientConfig
• CONFIG → SERVER_CONFIG for server-side usage
• Field access patterns updated (e.g., auth_server → auth.url)

Testing

• Changes were checked by building and running the dev environment based on the docker-compose.yml.

[ionicsolutions]

@philippschroeppel Can you please integrate this change as well? a243b99 It solves #8

You should be able to cherry-pick this commit to your branch. It will also give you a way to test your changes a bit more.

[philippschroeppel]

@ionicsolutions Unfortunately I was not able to cherry-pick the commit. I think the most elegant / idiomatic approach to enforcing unique server names (cf. #8) is to store client-side server configs in a hashmap.