2.0: Use gp247_namespace() for the developer-override convention in routes

Replaces the copy-pasted file_exists(app_path(...)) ? App\... : GP247\...
checks across Api and Front routes (admin, auth, brand, cart, category,
checkout, customer, product) with the shared gp247_namespace() helper from
gp247/core (requires core >= this change). No behavior change when no
override exists.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>