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>

server/internal/modules/auth/session_context.go

@@ -0,0 +1,24 @@
+package auth
+
+import "context"
+
+type requestMetadataContextKey struct{}
+type sessionIDContextKey struct{}
+
+func ContextWithRequestMetadata(ctx context.Context, metadata RequestMetadata) context.Context {
+	return context.WithValue(ctx, requestMetadataContextKey{}, metadata)
+}
+
+func RequestMetadataFromContext(ctx context.Context) RequestMetadata {
+	metadata, _ := ctx.Value(requestMetadataContextKey{}).(RequestMetadata)
+	return metadata
+}
+
+func ContextWithSessionID(ctx context.Context, sessionID string) context.Context {
+	return context.WithValue(ctx, sessionIDContextKey{}, sessionID)
+}
+
+func SessionIDFromContext(ctx context.Context) string {
+	sessionID, _ := ctx.Value(sessionIDContextKey{}).(string)
+	return sessionID
+}