feat(auth): add device session tracking with configurable limits

Track authenticated sessions per device (parsing user-agent for desktop
vs mobile via mileusna/useragent), enforce configurable concurrent device
limits (Auth.DeviceLimits), and surface device status and "remove other
devices" management in the account dialog. Adds device/session context to
the auth module stores and exposes limits through the API.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

deploy/config/api.docker.yaml

@@ -42,6 +42,9 @@ Auth:
   RefreshTTLSeconds: 2592000
   CodeTTLSeconds: 600
   DevReturnCode: true
+  DeviceLimits:
+    Desktop: 2
+    Mobile: 1
   Email:
     Driver: none # none|smtp
     Host: ""