feat(drill): Drill submission flow using stub runner; update DrillItem and redirect to next due

[ArchILLtect]

Problem / context

DrillService and ChallengeRunService exist, but there is no user-facing Drill flow to submit code and advance the due queue.

Proposed solution

• Add DrillController with endpoints:
  • GET /drill — show queue summary (top N) with CTA
  • GET /drill/next — resolve next due and redirect to /drill/{id}, or show friendly empty state
  • GET /drill/{id} — solve page with prompt, language select (java), and code textarea
  • POST /drill/{id}/submit — run stub, map to Outcome, DrillService.recordOutcome, flash message, redirect to /drill/next
• Views (JSP): drill/queue.jsp and drill/solve.jsp
• Show flash outcome after submit; add brief tip explaining the stub runner behavior.

In scope / Out of scope

In scope

• Controller, JSPs, wiring to ChallengeRunService and DrillService, flash messages, empty-queue UX
  Out of scope
• Real code execution or sandboxing, advanced editor features (tracked separately)

Acceptance criteria

• /drill shows a due summary with CTA to continue
• /drill/next redirects to a due challenge or shows a friendly message if none
• /drill/{id} renders solve JSP with prompt and form
• POST /drill/{id}/submit persists Submission, updates DrillItem, redirects to next due, shows flash outcome

Area

area:service

Dependencies / related issues

Uses existing DrillService

Checklist

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