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/api_key.rb

@@ -0,0 +1,66 @@
+class ApiKey < ApplicationRecord
+  belongs_to :user
+  belongs_to :application
+
+  before_validation :generate_token, on: :create
+
+  validates :name, presence: true
+  validates :token_hmac, presence: true, uniqueness: true
+  validate :application_must_be_forward_auth
+  validate :user_must_have_access
+
+  scope :active, -> { where(revoked_at: nil).where("expires_at IS NULL OR expires_at > ?", Time.current) }
+  scope :revoked, -> { where.not(revoked_at: nil) }
+
+  attr_accessor :plaintext_token
+
+  def self.find_by_token(plaintext_token)
+    return nil if plaintext_token.blank?
+
+    token_hmac = compute_token_hmac(plaintext_token)
+    find_by(token_hmac: token_hmac)
+  end
+
+  def self.compute_token_hmac(plaintext_token)
+    OpenSSL::HMAC.hexdigest("SHA256", TokenHmac::KEY, plaintext_token)
+  end
+
+  def expired?
+    expires_at.present? && expires_at <= Time.current
+  end
+
+  def revoked?
+    revoked_at.present?
+  end
+
+  def active?
+    !expired? && !revoked?
+  end
+
+  def revoke!
+    update!(revoked_at: Time.current)
+  end
+
+  def touch_last_used!
+    update_column(:last_used_at, Time.current)
+  end
+
+  private
+
+  def generate_token
+    self.plaintext_token ||= "clk_#{SecureRandom.urlsafe_base64(48)}"
+    self.token_hmac ||= self.class.compute_token_hmac(plaintext_token)
+  end
+
+  def application_must_be_forward_auth
+    if application && !application.forward_auth?
+      errors.add(:application, "must be a forward auth application")
+    end
+  end
+
+  def user_must_have_access
+    if user && application && !application.user_allowed?(user)
+      errors.add(:user, "does not have access to this application")
+    end
+  end
+end