feat(logger): migrate to `slog` and implement package-level wrappers

[joelfokou]

Problem

The current zap implementation requires calling logger.L() for every log entry which creates unnecessary boilerplate, exposes internal logger pointers and is not idiomatic.

Solution

• Replace uber-go/zap with the standard library log/slog.
• Unexport the internal logger instance.
• Provide package-level functions (Info, Error, Debug, Warn) to simplify the API.

Impact

Zap	Slog
logger.L().Info("msg", zap.Int("id", 1))	logger.Info("msg", "id", 1)
logger.L().Error("err", zap.Error(err))	logger.Error("err", "error", err)

Definition of Done

• zap dependency removed from go.mod.
• L() function removed in favor of direct package calls.
• All business logic updated to use the new simplified syntax.