🔑 Add Username Support for Users

[sensasi-delight]

Currently, users are identified only by their account information (e.g., email).
To improve personalization and discovery, we will add unique usernames that can be displayed on profiles, posts, comments, and leaderboards.

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✅ Tasks

• Database & Schema

  • Update User model in Prisma to include username field (unique, indexed).
  • Run Prisma migration for the new field.
  • Add validation (only alphanumeric + underscore, 3–20 chars).

• Authentication & Registration

  • Update signup flow to include username input.
  • Ensure username uniqueness is enforced at registration.
  • Add fallback username generator for OAuth (Google/GitHub).

• Profile & Settings

  • Display username on profile, posts, comments, and leaderboard.
  • Add option to update username in Profile Settings (with restrictions, e.g., max 1 change per 30 days).
  • Add friendly error messages for duplicate or invalid usernames.

• Routing & SEO

  • Update profile URLs to use /@username format.
  • Redirect old profile URLs (if any) to the new format.

• Testing

  • Add unit tests for username validation.
  • Add integration tests for signup and profile update flow.
  • Test OAuth signup with auto-generated username.

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🎯 Acceptance Criteria

• Users can choose a unique username during signup.
• Usernames are displayed consistently across the platform.
• Profile URLs follow the /@username format.
• Validation and uniqueness checks work reliably.