Enables server-to-server authentication for forward auth applications
(e.g., video players accessing WebDAV) where browser cookies aren't
available. API keys use clk_ prefixed tokens stored as HMAC hashes.
Bearer token auth is checked before cookie auth in /api/verify.
Invalid tokens return 401 JSON (no redirect). Requests without
bearer tokens fall through to existing cookie flow unchanged.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
