fix: resolve Instagram username via Facebook Graph API

[chubes4]

Summary

Fixes the studio UI showing @unknown for connected Instagram accounts even when auth is healthy.

Root cause

Our OAuth flow uses Facebook Login (exchange_for_long_lived_token_fb), which produces an FB-flavored access token. The previous username lookup hit graph.instagram.com/{user_id}?fields=username, which rejects those tokens with:

HTTP 400
{"error":{"message":"Invalid OAuth access token - Cannot parse access token","type":"OAuthException","code":190}}

The OAuth callback caught the resulting WP_Error and silently stored username = ''. Every reauth since the FB Login switch has been broken this way, but the failure never surfaced because:

1. Publishing still worked (publish hits FB Graph endpoints, which accept the token).
2. The empty username degraded gracefully to @unknown in the React UI.

Changes

• get_username_from_token() now hits graph.facebook.com/v18.0/{user_id}?fields=username, which accepts both FB Login and legacy IG Basic Display tokens. Verified live: returns {"username":"extrachill"}.
• get_instagram_user_id_from_facebook_token() → resolve_instagram_account_from_facebook_token() — same /me/accounts call, now returns array{id, username} so the OAuth callback can capture the username from the call we already make. Saves a round trip on the happy path.
• OAuth callback prefers the resolved username and only falls back to get_username_from_token() when missing (legacy IG Basic Display tokens). Lookup failures are now logged via datamachine_log so future regressions surface in debug.log instead of silently rendering @unknown.

Verification

Live probe against production token before merge:

GET graph.facebook.com/v18.0/17841400793710113?fields=username
→ HTTP 200 {"username":"extrachill","id":"17841400793710113"}

Production extrachill.com account already backfilled out-of-band as a one-shot data fix; this PR ensures the next reauth (and all future reauths) populate the field correctly.

Test plan

• ✅ php -l clean
• ✅ Live FB Graph probe returns username with the same token format the production install uses
• ⏳ Manual reauth on a staging IG account post-deploy to confirm callback path