Add AltumWebhookVerifier that validates inbound webhooks by checking
the User-Agent header (AltumCode sends unsigned webhooks via fire_and_forget).
Register verifier instances for all 4 product sources in AltumServiceProvider.
Co-Authored-By: Virgil <virgil@lethean.io>
Base API client for all 4 AltumCode products (analytics, biolinks,
pusher, socialproof). Shared REST conventions: Bearer auth, form-encoded
POST, Admin API (/admin-api/) and User API (/api/) with CRUD helpers.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>