Structured JSON logging (slog or zerolog)

[KingAkeem]

Summary

Introduce structured logging with fields per fetch and level control via flags.

Motivation

• Parseable logs for pipelines/SIEM
• Easier debugging at scale

Scope

• One JSON log per fetch: host, url, code, bytes, duration_ms, via=tor|direct, retry
• CLI flags: --log-level, --log-format=json|text

Acceptance Criteria

• Logs are valid JSON when requested
• Log level filtering works across all packages

Tasks

• Add logger abstraction (slog/zerolog)
• Replace fmt prints
• Document log options in README