fix: resolve issues #360, #361, #362 - grading retries, stale leaderboard, idempotent badges

[eulami]

Resolve issues #360, #361, #362 — Grading retries, stale leaderboard, idempotent badges

Issues Closed

• Closes **Enhancement: add grading retries with backoff for external evaluation providers** #360 – Enhancement: add grading retries with backoff for external evaluation providers
• Closes **Bug: leaderboard rank updates are stale during high-volume submissions** #361 – Bug: leaderboard rank updates are stale during high-volume submissions
• Closes **Enhancement: add badge awarding rules that are conflict-safe and idempotent** #362 – Enhancement: add badge awarding rules that are conflict-safe and idempotent

---

Summary of Changes

Issue #360 — Grading retries with backoff

Problem: External graders occasionally fail transiently. Submissions were failing immediately without retry.

Solution:

• config.module.ts: Added env var schema for GRADING_MAX_RETRIES, GRADING_RETRY_BASE_DELAY_MS, GRADING_RETRY_MAX_DELAY_MS with sensible defaults.
• grading-job.service.ts: Replaced hardcoded backoff with config-driven exponential backoff capped at maxDelayMs. Added notifyGradingFailure() to send in-app notifications when a grading job permanently exhausts all retries.
• challenges.service.ts: Added evaluateWithRetry() method that enqueues failed external evaluations into GradingJobService for retry instead of immediately failing.
• notifications.service.ts: Added sendGradingFailureAlert() method respecting user preference.

Issue #361 — Stale leaderboard rank updates

Problem: Leaderboard displayed stale/outdated scores using static sample data with no update mechanism.

Solution:

• leaderboard.service.ts: Added configurable cache with LEADERBOARD_CACHE_TTL_MS (default 30s). Added markStale() method. buildLeaderboard() now uses real SubmissionService data with time-range filtering.

Issue #362 — Conflict-safe, idempotent badge awarding

Problem: Badge issuance could create duplicates or inconsistent state when the same achievement was processed multiple times concurrently.

Solution:

• badges.service.ts: Added awardedBadgeSet for O(1) atomic conflict detection. awardBadge() is now idempotent: duplicate calls silently return existing badges. On first award, fires in-app notification + BADGE_EARNED analytics event.
• rewards.service.ts: Integrates badge awarding on streak/level milestones.

Module Wiring

• Updated challenges.module.ts, jobs.module.ts, badges.module.ts, rewards.module.ts, leaderboard.module.ts, and app.module.ts with proper module imports.

---

Validation

• All 16 modified files compile with zero type errors.
• Code review confirmed no critical issues - idempotent badges, clean module imports, correct backoff logic.

[MaryammAli]

LGTM