refactor(api): extract 8 content domains into api/*.ts slices (R7.5 S1)

Move feed, channels, sync, playlists, tags, notifications, saved-views and
quota out of the lib/api.ts god object into per-domain slice modules, each
owning its own types + method slice and spread into the façade. The `api`
object shape and all 54 call sites are unchanged; account/auth/setup/admin
stay inline until S2.

Reunites the interleaved tags read/CRUD that motivated the epic.

frontend/src/lib/api/channels.ts

@@ -0,0 +1,138 @@
+// Channel manager (subscribed + discovery), the channel About/detail page, ephemeral exploration
+// of un-subscribed channels, and the per-user channel blocklist. Only touches core's req.
+import { req } from "./core";
+
+interface ChannelLink {
+  title: string | null;
+  url: string;
+}
+
+export interface ChannelDetail {
+  id: string;
+  title: string | null;
+  handle: string | null;
+  description: string | null;
+  thumbnail_url: string | null;
+  banner_url: string | null;
+  subscriber_count: number | null;
+  video_count: number | null;
+  total_view_count: number | null;
+  published_at: string | null;
+  external_links: ChannelLink[];
+  country: string | null;
+  default_language: string | null;
+  topic_categories: string[];
+  keywords: string | null;
+  subscribed: boolean;
+  explored: boolean;
+  blocked: boolean;
+  stored_videos: number;
+  deep_requested: boolean;
+  backfill_done: boolean;
+  from_explore: boolean;
+  details_synced: boolean;
+}
+
+export interface ExploreResult {
+  channel_id: string;
+  ingested: number;
+  next_page_token: string | null;
+}
+
+export interface MyStatus {
+  channels_total: number;
+  channels_details_synced: number;
+  channels_recent_synced: number;
+  channels_deep_done: number;
+  channels_recent_pending: number;
+  channels_deep_pending: number;
+  channels_deep_requested: number;
+  deep_pending_count: number;
+  deep_eta_seconds: number;
+  my_videos: number;
+  total_videos: number;
+  quota_used_today: number;
+  quota_remaining_today: number;
+  paused: boolean;
+  sync_active: boolean; // a channel-sync job (backfill/rss) is running right now
+}
+
+export interface ManagedChannel {
+  id: string;
+  title: string | null;
+  handle: string | null;
+  thumbnail_url: string | null;
+  subscriber_count: number | null;
+  video_count: number | null;
+  description: string | null;
+  stored_videos: number;
+  last_video_at: string | null;
+  total_duration_seconds: number;
+  count_normal: number;
+  count_short: number;
+  count_live: number;
+  priority: number;
+  hidden: boolean;
+  deep_requested: boolean;
+  deep_in_queue: boolean;
+  tag_ids: number[];
+  details_synced: boolean;
+  recent_synced: boolean;
+  backfill_done: boolean;
+}
+
+// A channel that shows up in the user's playlists but that they don't subscribe to —
+// surfaced in the Channel manager's Discovery tab so they can subscribe in one click.
+export interface DiscoveredChannel {
+  id: string;
+  title: string | null;
+  handle: string | null;
+  thumbnail_url: string | null;
+  subscriber_count: number | null;
+  video_count: number | null;
+  description: string | null;
+  playlist_video_count: number; // how many of the user's playlist videos are from here
+  playlist_count: number; // across how many of their playlists
+  details_synced: boolean;
+}
+
+export const channelsApi = {
+  myStatus: (): Promise<MyStatus> => req("/api/sync/my-status"),
+  channels: (): Promise<ManagedChannel[]> => req("/api/channels"),
+  updateChannel: (
+    id: string,
+    patch: { priority?: number; hidden?: boolean; deep_requested?: boolean },
+  ) => req(`/api/channels/${id}`, { method: "PATCH", body: JSON.stringify(patch) }),
+  resetChannelBackfill: (id: string) =>
+    req(`/api/channels/${id}/reset-backfill`, { method: "POST" }),
+  deepAll: (on = true): Promise<{ updated: number; deep_requested: boolean }> =>
+    req(`/api/sync/deep-all?on=${on}`, { method: "POST" }),
+  attachChannelTag: (id: string, tagId: number) =>
+    req(`/api/channels/${id}/tags`, { method: "POST", body: JSON.stringify({ tag_id: tagId }) }),
+  detachChannelTag: (id: string, tagId: number) =>
+    req(`/api/channels/${id}/tags/${tagId}`, { method: "DELETE" }),
+  unsubscribeChannel: (id: string) => req(`/api/channels/${id}/subscription`, { method: "DELETE" }),
+  discoveredChannels: (): Promise<DiscoveredChannel[]> => req("/api/channels/discovery"),
+  subscribeChannel: (id: string) => req(`/api/channels/${id}/subscribe`, { method: "POST" }),
+  // --- channel page (About detail + ephemeral exploration of un-subscribed channels) ---
+  channelDetail: (id: string): Promise<ChannelDetail> => req(`/api/channels/${id}`),
+  // Open an un-subscribed channel for browsing: ingest a page of its uploads (newest-first,
+  // marked via_explore) + record the exploration. `pageToken` continues into older uploads
+  // ("load more"). `quiet` so a quota-reserve 429 / load error renders inline on the page.
+  exploreChannel: (id: string, pageToken?: string | null): Promise<ExploreResult> => {
+    const p = new URLSearchParams();
+    if (pageToken) p.set("page_token", pageToken);
+    const qs = p.toString();
+    return req(
+      `/api/channels/${id}/explore${qs ? `?${qs}` : ""}`,
+      { method: "POST" },
+      { quiet: true },
+    );
+  },
+  // --- per-user channel blocklist (excluded from live search + feed/explore) ---
+  blockedChannels: (): Promise<
+    { id: string; title: string | null; handle: string | null; thumbnail_url: string | null }[]
+  > => req("/api/channels/blocked/list"),
+  blockChannel: (id: string) => req(`/api/channels/${id}/block`, { method: "POST" }),
+  unblockChannel: (id: string) => req(`/api/channels/${id}/block`, { method: "DELETE" }),
+};