feat(log): add configurable log levels

[0dillon]

Summary

The logging package currently logs at a fixed level. We should allow operators to configure the log level via an environment variable (e.g. SEP10_LOG_LEVEL).

Acceptance Criteria

• Add SEP10_LOG_LEVEL to internal/config.
• Update internal/log to respect the configured level (debug, info, warn, error).
• Validate config correctly defaults to info.

Tech Stack

Go, log/slog