Cap device-code poll interval and sweep expired device codes

Two more fixes from the device-flow security review:

slow_down interval grew without bound (OidcController):
- Each too-fast poll bumped the persisted interval by 5s with no ceiling, so a
  client polling slightly fast — or an attacker spamming a known device_code —
  could balloon it (5→10→15→…) past the 10-minute expiry and starve a legitimate
  client of its token. Clamp the bump to OidcDeviceCode::MAX_INTERVAL (30s); a
  client polling at the advertised interval is never throttled.

Expired device codes accumulated forever (OidcTokenCleanupJob):
- Anonymous callers can create device codes via /oauth/device_authorization, and
  expired/denied/abandoned rows were never reaped. Sweep rows past expiry (with a
  1-hour grace to stay clear of in-flight redemption); redeemed codes are already
  destroyed at token issuance.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016Q4ATZHoMCWqvSpE2yYoie

app/controllers/oidc_controller.rb

@@ -611,9 +611,12 @@ class OidcController < ApplicationController
 
       if device_code.pending?
         # Enforce the polling interval; too-frequent polls get slow_down, and the
-        # client is expected to add 5s to its interval (RFC 8628 §3.5).
+        # client is expected to add 5s to its interval (RFC 8628 §3.5). The bump is
+        # capped at MAX_INTERVAL so a persistently fast poller can't grow it without
+        # bound and starve a legitimate client before the code expires.
         if device_code.last_polled_at && (Time.current - device_code.last_polled_at) < device_code.interval
-          device_code.update!(interval: device_code.interval + 5, last_polled_at: Time.current)
+          bumped_interval = [device_code.interval + OidcDeviceCode::INTERVAL_INCREMENT, OidcDeviceCode::MAX_INTERVAL].min
+          device_code.update!(interval: bumped_interval, last_polled_at: Time.current)
           render json: {error: "slow_down"}, status: :bad_request
         else
           device_code.update!(last_polled_at: Time.current)

app/jobs/oidc_token_cleanup_job.rb

@@ -25,5 +25,15 @@ class OidcTokenCleanupJob < ApplicationJob
     old_auth_codes = OidcAuthorizationCode.where("created_at < ?", 7.days.ago)
     deleted_count = old_auth_codes.delete_all
     Rails.logger.info "OIDC Token Cleanup: Deleted #{deleted_count} old authorization codes"
+
+    # Delete expired device codes (RFC 8628). They have a ~10 minute TTL and no
+    # audit value; a redeemed code is already destroyed at token issuance. Once
+    # expired a code can never be redeemed, so this single expiry sweep clears the
+    # expired, denied, and abandoned rows that would otherwise accumulate forever
+    # (anonymous callers can create them via /oauth/device_authorization). The
+    # short grace keeps this clear of any in-flight redemption near expiry.
+    expired_device_codes = OidcDeviceCode.where("expires_at < ?", 1.hour.ago)
+    deleted_count = expired_device_codes.delete_all
+    Rails.logger.info "OIDC Token Cleanup: Deleted #{deleted_count} expired device codes"
   end
 end

app/models/oidc_device_code.rb

@@ -17,6 +17,14 @@ class OidcDeviceCode < ApplicationRecord
 
   STATUSES = %w[pending approved denied].freeze
 
+  # Polling interval, in seconds. The token endpoint bumps the persisted interval
+  # by INTERVAL_INCREMENT on each too-fast poll (RFC 8628 §3.5 slow_down), but
+  # clamps it to MAX_INTERVAL so a client polling slightly fast — or an attacker
+  # spamming a known device_code — can't balloon it past the expiry window and
+  # starve a well-behaved client of its token.
+  INTERVAL_INCREMENT = 5
+  MAX_INTERVAL = 30
+
   attr_accessor :plaintext_device_code
 
   before_validation :generate_device_code, on: :create