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/lib/useChannelTagActions.ts

@@ -0,0 +1,43 @@
+import { useQueryClient } from "@tanstack/react-query";
+import { useTranslation } from "react-i18next";
+import { api } from "./api";
+import { qk } from "./queryKeys";
+import { notify } from "./notifications";
+
+/** The shared channel-tag mutation flow used by both the channel manager and the channel detail
+ *  page: optimistic attach/detach + a create-then-attach shortcut, with the cache-shape-specific
+ *  optimistic update injected (the manager maps over a ManagedChannel[]; the detail patches one
+ *  ChannelDetail). Keeps the two call sites from drifting. */
+export function useChannelTagActions(opts: {
+  /** Apply the optimistic tag change to whichever cache the caller owns. `attach` true = add. */
+  optimisticToggle: (channelId: string, tagId: number, attach: boolean) => void;
+  /** Called after the attach/detach lands (e.g. sync a sibling cache). Optional. */
+  onSuccess?: (channelId: string) => void;
+  /** Called if the attach/detach fails — revert by refetching the owning cache. */
+  onError: (channelId: string) => void;
+}) {
+  const qc = useQueryClient();
+  const { t } = useTranslation();
+
+  const toggleTag = (channelId: string, tagId: number, hasTag: boolean) => {
+    opts.optimisticToggle(channelId, tagId, !hasTag);
+    const call = hasTag
+      ? api.detachChannelTag(channelId, tagId)
+      : api.attachChannelTag(channelId, tagId);
+    call.then(() => opts.onSuccess?.(channelId)).catch(() => opts.onError(channelId));
+  };
+
+  // Create a brand-new personal tag and attach it in one step (so tagging doesn't require the
+  // top-of-page "Manage tags" dialog first). Refresh the tag catalog so it appears everywhere.
+  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") }));
+  };
+
+  return { toggleTag, createAndAttachTag };
+}