Refactor: replace global `config.C` with idiomatic getter

[joelfokou]

Problem

The current implementation uses a global exported variable var C Config. This can cause a few issues

• The global variable can be overwritten by any package importing config.
• Doesn't respect naming clarity according to "Effective Go" principles.
• config.C is often mistaken for CGo calls.

Proposed Solution

Refactor the config package to hide the global state and expose it through a read-only getter.

Changes in config/config.go

• Change var C Config to var instance Config.
• Implement a Get() function.

func Get() Config {
    return instance
}

• Ensure Load() populates the unexported instance and returns (*Config, error) for better accessibility throughout project.

Project-wide Impact

Replace all instances of config.C with config.Get().

Current	Refactored
config.C.LogLevel	config.Get().LogLevel
config.C.Paths.Database	config.Get().Paths.Database

Definition of Done

• config.C is no longer exported.
• No direct modification of configuration is possible from external packages.
• All existing tests run successfully go test ./....
• Application compiles successfully with config.Get().