feat(channels): discover & subscribe to channels from playlists

Add a "Discover from playlists" tab to the Channel manager that lists
channels appearing in the user's playlists they don't subscribe to, with
a one-click Subscribe.

- GET /api/channels/discovery: local join (playlist_items -> videos ->
  channels) minus the user's subscriptions and their own channel. Enriches
  stub channels' metadata up front (title/thumbnail/subscriber count via the
  API key) so the user can judge a channel before subscribing; videos are
  not pulled (the scheduler picks those up).
- POST /api/channels/{id}/subscribe: write-scope gated, subscriptions.insert
  + local Subscription with the returned resource id.
- YouTubeClient.insert_subscription / get_my_channel_id.
- users.yt_channel_id (migration 0019) caches the user's own channel id so
  discovery can exclude it.
- Frontend: ChannelDiscovery DataTable, Channels tab toggle (persisted),
  api methods, trilingual strings. Subscribe ships a typed notification
  payload (ChannelSubscribedMeta) for the inbox to act on.

frontend/src/components/ChannelDiscovery.tsx

@@ -0,0 +1,179 @@
+import { useTranslation } from "react-i18next";
+import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
+import { ExternalLink, UserPlus } from "lucide-react";
+import { api, HttpError, type DiscoveredChannel } from "../lib/api";
+import { formatViews } from "../lib/format";
+import { notify } from "../lib/notifications";
+import Tooltip from "./Tooltip";
+import Avatar from "./Avatar";
+import DataTable, { type Column } from "./DataTable";
+
+// The Channel manager's "Discovery" tab: channels that turn up in the user's playlists but
+// that they don't subscribe to. Subscribing here only creates the subscription + enriches
+// the channel's metadata — the scheduler pulls its videos on its next run (see backend).
+export default function ChannelDiscovery({
+  canWrite,
+  onOpenWizard,
+}: {
+  canWrite: boolean;
+  onOpenWizard: () => void;
+}) {
+  const { t } = useTranslation();
+  const qc = useQueryClient();
+
+  const query = useQuery({
+    queryKey: ["discovered-channels"],
+    queryFn: api.discoveredChannels,
+  });
+
+  const subscribe = useMutation({
+    mutationFn: (c: DiscoveredChannel) => api.subscribeChannel(c.id),
+    onSuccess: (_data, c) => {
+      // The channel moves from "discovery" to "subscribed", and its videos will start
+      // arriving — refresh both lists plus the per-user status.
+      qc.invalidateQueries({ queryKey: ["discovered-channels"] });
+      qc.invalidateQueries({ queryKey: ["channels"] });
+      qc.invalidateQueries({ queryKey: ["my-status"] });
+      const name = c.title ?? c.id;
+      // Name the channel and ship a typed payload so the inbox can offer "open in the
+      // Channel manager" / "open on YouTube" links — kept after reload, when live
+      // callbacks are gone (see ChannelSubscribedMeta).
+      notify({
+        level: "success",
+        title: t("channels.discovery.subscribedTitle"),
+        message: t("channels.discovery.subscribedBody", { name }),
+        meta: { kind: "channel-subscribed", channelId: c.id, channelName: name },
+      });
+    },
+    onError: (err: unknown) => {
+      // A 403 means the user hasn't granted the write scope — offer to connect instead of
+      // a vague failure (mirrors the subscriptions tab).
+      if (err instanceof HttpError && err.status === 403) {
+        notify({
+          level: "error",
+          message: t("channels.notify.needYouTube"),
+          action: { label: t("channels.notify.connect"), onClick: onOpenWizard },
+        });
+      } else {
+        notify({ level: "error", message: t("channels.discovery.subscribeFailed") });
+      }
+    },
+  });
+
+  const rows = query.data ?? [];
+
+  const columns: Column<DiscoveredChannel>[] = [
+    {
+      key: "channel",
+      header: t("channels.cols.channel"),
+      sortable: true,
+      sortValue: (c) => (c.title ?? c.id).toLowerCase(),
+      filter: { kind: "text", get: (c) => `${c.title ?? ""} ${c.handle ?? ""}` },
+      cardPrimary: true,
+      render: (c) => <DiscoveryNameCell c={c} />,
+    },
+    {
+      key: "subs",
+      header: t("channels.cols.subs"),
+      align: "right",
+      nowrap: true,
+      sortable: true,
+      sortValue: (c) => c.subscriber_count ?? -1,
+      render: (c) => (
+        <span className="text-muted tabular-nums">
+          {c.subscriber_count != null ? formatViews(c.subscriber_count) : "—"}
+        </span>
+      ),
+    },
+    {
+      key: "inPlaylists",
+      header: t("channels.discovery.cols.inPlaylists"),
+      align: "right",
+      nowrap: true,
+      sortable: true,
+      sortValue: (c) => c.playlist_video_count,
+      render: (c) => (
+        <Tooltip
+          hint={t("channels.discovery.cols.inPlaylistsHint", {
+            videos: c.playlist_video_count,
+            playlists: c.playlist_count,
+          })}
+        >
+          <span className="text-muted tabular-nums cursor-help">
+            {c.playlist_video_count} / {c.playlist_count}
+          </span>
+        </Tooltip>
+      ),
+    },
+    {
+      key: "actions",
+      header: t("channels.cols.actions"),
+      align: "right",
+      nowrap: true,
+      width: "120px",
+      cardLabel: false,
+      render: (c) => (
+        <Tooltip
+          hint={
+            canWrite
+              ? t("channels.discovery.subscribeHint")
+              : t("channels.discovery.needWriteHint")
+          }
+        >
+          <button
+            onClick={() => subscribe.mutate(c)}
+            disabled={!canWrite || subscribe.isPending}
+            className="glass-card glass-hover inline-flex items-center gap-1.5 px-2.5 py-1.5 rounded-lg text-xs disabled:opacity-50 transition"
+          >
+            <UserPlus className="w-3.5 h-3.5" />
+            {t("channels.discovery.subscribe")}
+          </button>
+        </Tooltip>
+      ),
+    },
+  ];
+
+  return (
+    <div className="px-4 pb-4 pt-3 max-w-7xl mx-auto">
+      <p className="text-xs text-muted mb-4 leading-relaxed">
+        {t("channels.discovery.intro")}
+      </p>
+      {query.isLoading ? (
+        <div className="text-muted py-8">{t("channels.discovery.loading")}</div>
+      ) : (
+        <DataTable
+          rows={rows}
+          columns={columns}
+          rowKey={(c) => c.id}
+          persistKey="siftlode.channelDiscoveryTable"
+          controlsPosition="top"
+          emptyText={t("channels.discovery.empty")}
+        />
+      )}
+    </div>
+  );
+}
+
+function DiscoveryNameCell({ c }: { c: DiscoveredChannel }) {
+  const { t } = useTranslation();
+  const ytUrl = c.handle
+    ? `https://www.youtube.com/@${c.handle.replace(/^@/, "")}`
+    : `https://www.youtube.com/channel/${c.id}`;
+  return (
+    <div className="flex items-center gap-2 min-w-0">
+      <Avatar src={c.thumbnail_url} fallback={c.title ?? ""} className="w-8 h-8 rounded-full shrink-0" />
+      <span className="text-sm font-medium truncate min-w-0">{c.title ?? c.id}</span>
+      <Tooltip hint={t("channels.row.openOnYouTube")}>
+        <a
+          href={ytUrl}
+          target="_blank"
+          rel="noopener noreferrer"
+          className="text-muted hover:text-accent shrink-0"
+          aria-label={t("channels.row.openOnYouTube")}
+        >
+          <ExternalLink className="w-3.5 h-3.5" />
+        </a>
+      </Tooltip>
+    </div>
+  );
+}