Verify GitHub social link

Verify ownership of a GitHub social link using an OAuth 2.0 access token.

Flow:

1. Client obtains a GitHub OAuth 2.0 access token via authorization code flow
2. Client sends the token to this endpoint
3. API validates the token against GitHub's GET /user endpoint
4. If the returned username matches the account's existing GitHub social link, it's marked as verified

Prerequisites:

• Account must have a GitHub social link already added (via POST /v1/account/social-links)
• Client must have a valid GitHub OAuth 2.0 access token

Idempotent: Calling this on an already-verified GitHub link returns success.

Authentication: Authorization: Bearer <CLIENT_JWT>