fix: mark-as-read for items from orphan channels

Items from channels with userId: null (created during earlier setup)
appeared in the unified timeline but had no _channelUid, causing the
mark-read JS handler to silently abort. Fall back to channelId (MongoDB
ObjectId) when channelUid is unavailable, and resolve it server-side
via getChannelById.

Confab-Link: http://localhost:8080/sessions/4d40ef89-a713-48c1-b4ed-0ffafca25677

lib/controllers/timeline.js

@@ -6,7 +6,7 @@
 import { IndiekitError } from "@indiekit/error";
 
 import { proxyItemImages } from "../media/proxy.js";
-import { getChannel } from "../storage/channels.js";
+import { getChannel, getChannelById } from "../storage/channels.js";
 import {
   getTimelineItems,
   markItemsRead,
@@ -72,8 +72,16 @@ export async function action(request, response) {
 
   validateChannel(channel);
 
-  // Verify channel exists
-  const channelDocument = await getChannel(application, channel, userId);
+  // Verify channel exists — try by UID first, fall back to ObjectId
+  // (timeline view may send ObjectId string for items from orphan channels)
+  let channelDocument = await getChannel(application, channel, userId);
+  if (!channelDocument) {
+    try {
+      channelDocument = await getChannelById(application, channel);
+    } catch {
+      // Invalid ObjectId format — channel string is not a valid ObjectId
+    }
+  }
   if (!channelDocument) {
     throw new IndiekitError("Channel not found", {
       status: 404,