feat: daily challenge and streak system

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Description

Closes #148

This PR implements the requested Daily Challenge and Streak Tracking System for the backend API. It establishes an automated @nestjs/schedule cron job that rotates the active puzzle daily while incorporating comprehensive streak increments, grace period allowances, and dynamic bonus point rewarding systems.

Features Added

• Database Schemata:
  • DailyChallenge: Represents a puzzle specific to a UTC day with a scaled difficulty modifier and an explicit active flag.
  • DailyChallengeCompletion: A junction entity tracking a user's completion data, including individual score, time spent, and streak bonuses awarded.
• Rotation Cron Job:
  • Selects rotating puzzles daily based on the weekday (e.g., Easy on Mon/Tue, Medium on Wed/Thu, Hard on Fri, Expert on Sat/Sun).
  • Filters out recently used puzzles (last 30 days) to guarantee a diverse play experience.
• Streak & Bonus System:
  • Leverages the existing UserStreak entity. A continuous completion record increments the currentStreak resulting in increasing point multiplier rewards (e.g. +5% extra points per consecutive day, capping at 50% total bonus limit).
  • Automatically zeroes the streak history on a missed day unless rescued by the player's active streakRecoveryGracePeriodEnd.
• REST Endpoints:
  • GET /daily-challenges/today - Retrieve the active daily puzzle and completion status.
  • POST /daily-challenges/:id/complete - Submits scores and returns active streaks + bonus points awarded.
  • GET /daily-challenges/history - Fetches the last 30 puzzle completions.

Testing

Comprehensive unit testing functionality built within Jest:

• ✅ should create a new streak on first completion
• ✅ should increment streak when completed on consecutive days
• ✅ should reset streak when a day is missed without a grace period
• ✅ should maintain streak when a day is missed but within grace period

Self-Review Checklist

• Code strictly matches all requirements mapped out in issue Daily Challenge and Streak System #148.
• Unrelated code, folders, variables, and formats were not refactored or 'cleaned up'.
• No dependencies were modified outside of standard NextJS behavior.
• Testing verifiably handles logic for Streak flow boundaries.
• Codebase compilation runs cleanly for daily-challenges.