⚡ Bolt: Optimize checkAchievements performance

[google-labs-jules]

⚡ Bolt: Batch database operations in checkAchievements

💡 What: Refactored checkAchievements in gamificationController.js to batch database operations and aggregate XP awards.
🎯 Why: The previous implementation suffered from an N+1 query problem, executing multiple sequential database writes and XP updates for each unlocked achievement.
📊 Impact: Reduces database round-trips from O(N) to O(1) (constant number of await steps) for N unlocked achievements. Specifically, User.addXP is called once instead of N times.
🔬 Measurement: Added backend/tests/gamificationPerformance.test.js which verifies that User.addXP is called exactly once when multiple achievements are unlocked simultaneously.

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PR created automatically by Jules for task 15797217165473699657 started by @4444J99

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[4444J99]

Engaging as a real, still-open perf want (not falsely marked done): I confirmed the N+1 on current main — gamificationController.checkAchievements loops over unlocked achievements and runs 3 sequential awaits per achievement (INSERT user_achievements, user.addXP → user.save, INSERT notifications, around lines 133–173). The safe distilled form is to aggregate XP into a single addXP and bulk-insert the achievement + notification rows once. I'm not blind-merging this stale/conflicting draft because a DB-write refactor needs a live-DB integration test to verify correctness (achievement unlocking must not regress). Keeping this open as the anchor for that verified distillation — never closed.