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/ChannelPage.tsx

@@ -26,6 +26,7 @@ import { api, type FeedFilters, type Me, type ChannelDetail } from "../lib/api";
 import type { FeedView } from "../lib/feedView";
 import { channelYouTubeUrl, formatViews } from "../lib/format";
 import { ChannelTagsControl } from "./ChannelTags";
+import { useChannelTagActions } from "../lib/useChannelTagActions";
 
 // A dedicated channel page: an "About"-style header (banner, avatar, stats, subscribe) over the
 // channel's videos (the catalog filtered to this channel). For an un-subscribed channel it
@@ -167,38 +168,24 @@ export default function ChannelPage({
     onError: (err) => notifyYouTubeActionError(err, t("channel.fullHistoryFailed")),
   });
 
-  // Personal tags, editable right here (the same control the channel manager uses). Optimistic on
-  // this channel's detail cache; a failed attach/detach reverts by refetching. Invalidate the manager
-  // list too so a tag added here shows there. createTag mirrors the manager's create-and-attach.
+  // Personal tags, editable right here (the same control + flow the channel manager uses). Optimistic
+  // on this channel's detail cache; a failed attach/detach reverts by refetching, and a success
+  // invalidates the manager list so a tag added here shows there too.
   const tagsQuery = useQuery({ queryKey: qk.tags(), queryFn: api.tags });
   const userTags = (tagsQuery.data ?? []).filter((tg) => !tg.system);
-  const toggleTag = (tagId: number) => {
-    if (!ch) return;
-    const hasTag = ch.tag_ids.includes(tagId);
-    qc.setQueryData<ChannelDetail>(qk.channel(channelId), (old) =>
-      old
-        ? {
-            ...old,
-            tag_ids: hasTag ? old.tag_ids.filter((x) => x !== tagId) : [...old.tag_ids, tagId],
-          }
-        : old,
-    );
-    const call = hasTag
-      ? api.detachChannelTag(channelId, tagId)
-      : api.attachChannelTag(channelId, tagId);
-    call
-      .then(() => qc.invalidateQueries({ queryKey: qk.channels() }))
-      .catch(() => qc.invalidateQueries({ queryKey: qk.channel(channelId) }));
-  };
-  const createAndAttachTag = (name: string) => {
-    api
-      .createTag({ name })
-      .then((tag) => {
-        qc.invalidateQueries({ queryKey: qk.tags() });
-        toggleTag(tag.id);
-      })
-      .catch(() => notify({ level: "error", message: t("channels.tags.createFailed") }));
-  };
+  const { toggleTag, createAndAttachTag } = useChannelTagActions({
+    optimisticToggle: (id, tagId, attach) =>
+      qc.setQueryData<ChannelDetail>(qk.channel(id), (old) =>
+        old
+          ? {
+              ...old,
+              tag_ids: attach ? [...old.tag_ids, tagId] : old.tag_ids.filter((x) => x !== tagId),
+            }
+          : old,
+      ),
+    onSuccess: () => qc.invalidateQueries({ queryKey: qk.channels() }),
+    onError: (id) => qc.invalidateQueries({ queryKey: qk.channel(id) }),
+  });
 
   const onUnsubscribe = async () => {
     const ok = await confirm({
@@ -452,8 +439,8 @@ export default function ChannelPage({
                 <ChannelTagsControl
                   userTags={userTags}
                   attachedIds={ch.tag_ids}
-                  onToggle={toggleTag}
-                  onCreate={createAndAttachTag}
+                  onToggle={(tagId) => toggleTag(channelId, tagId, ch.tag_ids.includes(tagId))}
+                  onCreate={(name) => createAndAttachTag(channelId, name)}
                 />
               )}
             </div>