Add structured logging with configurable levels

[sebastientaggart]

Summary

Current logging is basic print-style. Add structured logging (JSON format option) with configurable log levels via settings.

Tasks

• Use Python logging module consistently across all components
• Add log_level and log_format to settings (plain vs JSON)
• Include context in log records (agent_id, action_name, etc.)
• Log security-relevant events (auth failures, rejected requests)