feat(backend+frontend): add username-based public profile lookup endpoint and public learner profile page

[portableDD]

Description

GET /api/v1/users/:id/public requires a UUID that users do not know. The leaderboard shows usernames — clicking them should navigate to a public profile page. This issue adds username-based lookup on the backend and a public learner profile page on the frontend.

Background

• UsersService.getPublicProfile(id) takes UUID — no username-based lookup exists
• No GET /api/v1/users/by-username/:username endpoint
• Leaderboard usernames need to link to /learners/{username}

Implementation Guide

Backend: add GET /api/v1/users/by-username/:username with case-insensitive username lookup. Return { id, username, xp, badgesCount, coursesCompleted, avatarUrl, bio } — never email or walletAddress. Return 404 for non-existent username. Add Swagger decorators.

Frontend: create frontend/app/learners/[username]/page.tsx showing avatar, username, XP bar, badge count, courses completed, and earned badges grid. Wire to the new endpoint.

Update the leaderboard table (Issue 39) to link each username to /learners/{username}.

Acceptance Criteria

• GET /api/v1/users/by-username/:username returns public profile without PII
• Case-insensitive username lookup
• Returns 404 for non-existent username
• Response never exposes email or wallet address
• /learners/[username] renders public profile page
• Leaderboard usernames link to profile page
• Swagger decorators on the new backend endpoint

Complexity: High - 200 points
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