feat: use slog for structured logging

[icholy]

This switches the logging to use log/slog and adds a --log-level flag to control the level.

[gemini-code-assist]

Code Review

This pull request migrates the application's logging from the standard "log" package to "slog" to support structured logging. The changes involve updating log calls across "client.go", "http.go", and "main.go" to use structured keys and appropriate log levels, as well as introducing a new "-log-level" command-line flag for configuration. Feedback was provided to standardize the key naming for failure counts in "client.go" to ensure consistency and improve log searchability.