* feat(auth): proxy OIDC provider icons server-side (#1333)
Strict img-src CSP blocked external OIDC icon hosts on the login page.
Loosening CSP was rejected via the MakerWorld precedent, so icons are
proxied: admin sets icon_url, backend fetches and caches the bytes in a
deferred BLOB column, the SPA renders from a same-origin
/api/v1/auth/oidc/providers/{id}/icon endpoint.
