refactor(channels): shared tag-mutation hook + a channel_detail tag_ids test (R8 review)

Address the two remaining review findings on the tag work:
- extract useChannelTagActions — the optimistic attach/detach + create-and-attach flow that
  the channel manager and the channel detail page had copy-pasted; the cache-shape-specific
  optimistic update is injected (manager maps a ManagedChannel[]; detail patches one
  ChannelDetail), so the two stay in sync;
- add a DB-lane test asserting GET /api/channels/{id} returns exactly this user's personal
  tag links — not another user's, not the auto/system (user_id NULL) links.

frontend/src/components/Channels.tsx

@@ -28,6 +28,7 @@ import DataTable, { type Column } from "./DataTable";
 import ChannelLayoutGrid from "./ChannelLayoutGrid";
 import { LoadingState, StateMessage } from "./QueryState";
 import { ChannelTagsControl } from "./ChannelTags";
+import { useChannelTagActions } from "../lib/useChannelTagActions";
 import ViewSwitcher, { type ViewOption } from "./ViewSwitcher";
 import ColumnSortControl from "./ColumnSortControl";
 import Pager from "./Pager";
@@ -195,34 +196,22 @@ export default function Channels() {
   }, []);
 
   // Tagging is updated optimistically in place (no refetch of the whole channel list, which
-  // made the toggle feel ~2s slow); only revert by refetching if the server call fails.
-  const toggleTag = (id: string, tagId: number, hasTag: boolean) => {
-    qc.setQueryData<ManagedChannel[]>(qk.channels(), (old) =>
-      (old ?? []).map((ch) =>
-        ch.id === id
-          ? {
-              ...ch,
-              tag_ids: hasTag ? ch.tag_ids.filter((x) => x !== tagId) : [...ch.tag_ids, tagId],
-            }
-          : ch,
+  // made the toggle feel ~2s slow); only revert by refetching if the server call fails. The
+  // attach/detach + create-and-attach flow is shared with the channel detail page.
+  const { toggleTag, createAndAttachTag } = useChannelTagActions({
+    optimisticToggle: (id, tagId, attach) =>
+      qc.setQueryData<ManagedChannel[]>(qk.channels(), (old) =>
+        (old ?? []).map((ch) =>
+          ch.id === id
+            ? {
+                ...ch,
+                tag_ids: attach ? [...ch.tag_ids, tagId] : ch.tag_ids.filter((x) => x !== tagId),
+              }
+            : ch,
+        ),
       ),
-    );
-    const call = hasTag ? api.detachChannelTag(id, tagId) : api.attachChannelTag(id, tagId);
-    call.catch(() => qc.invalidateQueries({ queryKey: qk.channels() }));
-  };
-
-  // Create a brand-new personal tag from the row/card editor and attach it to this channel in one
-  // step (so tagging no longer requires the top-of-page "Manage tags" dialog first). Refresh the tag
-  // catalog so the new tag appears everywhere; attach optimistically via toggleTag.
-  const createAndAttachTag = (channelId: string, name: string) => {
-    api
-      .createTag({ name })
-      .then((tag) => {
-        qc.invalidateQueries({ queryKey: qk.tags() });
-        toggleTag(channelId, tag.id, false);
-      })
-      .catch(() => notify({ level: "error", message: t("channels.tags.createFailed") }));
-  };
+    onError: () => qc.invalidateQueries({ queryKey: qk.channels() }),
+  });
   const syncSubs = useMutation({
     mutationFn: () => api.syncSubscriptions(),
     onSuccess: (r: { subscriptions?: number }) => {