chore(logging): Structured logs and counters for submit events

[ArchILLtect]

Problem / context

Need minimal visibility into usage and outcomes without full analytics.

Proposed solution

• Log structured entries on submit: mode, challengeId, language, outcome, duration
• Optional in-memory counters per mode

In scope / Out of scope

In scope:

• Logging statements, small counter utility
  Out of scope:
• Metrics endpoint/UI

Acceptance criteria

• Logs include fields above
• Basic counters tracked (if implemented)

Area

area:logging

Dependencies / related issues

Practice and Drill submit handlers

Checklist

• I’ve checked existing issues for duplicates.
• I can help implement this and open a PR.