Verify Facebook social link

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

Flow:

1. Client obtains a Facebook OAuth 2.0 access token via authorization code flow
2. Client sends the token to this endpoint
3. API validates the token against Facebook's Graph API and retrieves the user's profile ID
4. Compares the Facebook numeric ID against the stored social link URL to prevent verifying someone else's profile
5. If valid, the account's Facebook social link is marked as verified

Prerequisites:

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

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

Authentication: Authorization: Bearer <CLIENT_JWT>