Add API keys / bearer tokens for forward auth

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>

app/models/user.rb

@@ -9,6 +9,7 @@ class User < ApplicationRecord
   has_many :application_user_claims, dependent: :destroy
   has_many :oidc_user_consents, dependent: :destroy
   has_many :webauthn_credentials, dependent: :destroy
+  has_many :api_keys, dependent: :destroy
 
   # Token generation for passwordless flows
   generates_token_for :invitation_login, expires_in: 24.hours do