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.

backend/tests/test_channel_tags.py

@@ -0,0 +1,54 @@
+"""DB-backed regression (R8 Part 2): GET /api/channels/{id} returns exactly THIS user's personal
+tag links for the channel — not another user's, and not the auto/system links (user_id NULL) —
+so the channel detail page can reuse the manager's tag editor. Would ship broken if the tag_ids
+query ever dropped its user_id filter.
+
+Uses the DB lane (`db` fixture) — skipped when no Postgres is reachable.
+"""
+from datetime import datetime, timezone
+
+from fastapi.testclient import TestClient
+
+import app.state as state
+from app.auth import current_user
+from app.db import get_db
+from app.main import app
+from app.models import Channel, ChannelTag, Tag, User
+
+
+def test_channel_detail_returns_only_this_users_tag_ids(db, monkeypatch):
+    # published_at set so channel_detail skips the YouTube-enrich path (no quota / network in tests).
+    ch = Channel(
+        id="UC_tags_test",
+        title="Tag Test Channel",
+        published_at=datetime(2020, 1, 1, tzinfo=timezone.utc),
+    )
+    db.add(ch)
+    me = User(email="me@example.com")
+    other = User(email="other@example.com")
+    db.add_all([me, other])
+    db.flush()
+    mine = Tag(user_id=me.id, name="Mine", category="other")
+    theirs = Tag(user_id=other.id, name="Theirs", category="other")
+    auto = Tag(user_id=None, name="English", category="language")  # a system/auto tag
+    db.add_all([mine, theirs, auto])
+    db.flush()
+    # My personal link, another user's link, and a system/auto link on the same channel — only MINE
+    # must come back for `me`.
+    db.add(ChannelTag(channel_id=ch.id, tag_id=mine.id, user_id=me.id, source="user"))
+    db.add(ChannelTag(channel_id=ch.id, tag_id=theirs.id, user_id=other.id, source="user"))
+    db.add(ChannelTag(channel_id=ch.id, tag_id=auto.id, user_id=None, source="auto"))
+    db.commit()
+
+    # The request gate 503s an unconfigured instance; flip the process-level flag (auto-restored).
+    monkeypatch.setattr(state, "_configured_cache", True)
+    app.dependency_overrides[get_db] = lambda: db
+    app.dependency_overrides[current_user] = lambda: me
+    try:
+        client = TestClient(app)  # no context-manager: skip lifespan (scheduler/HLS sweep)
+        resp = client.get(f"/api/channels/{ch.id}")
+    finally:
+        app.dependency_overrides.clear()
+
+    assert resp.status_code == 200, resp.text
+    assert resp.json()["tag_ids"] == [mine.id]

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>

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 }) => {

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 };
+}