feat: the extension picks a Discord source's posters from who posts in the channel, stored by id (#4488)

On a Discord channel page the Add panel lists who posted in the newest 200
messages, the creator the server is named for (or owned by) ticked. On a
channel FabledCurator already follows, the chip opens the same list with Save
and Open artist. The list is kept as user ids, so a rename never quietly stops
it matching; the name each was picked under rides beside it for display, and
the Subscriptions dialog shows it.

- DiscordClient.recent_posters + rank_posters (owner / name-matches-server;
  bots never suggested; it suggests, the operator ticks).
- GET/POST /api/extension/discord/posters; quick-add takes discord_authors.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

extension/content/content-script.js

@@ -81,6 +81,13 @@
     const probe = currentProbe;
 
     if (probe?.state === 'source_match') {
+      // A followed Discord channel opens its poster list (#4488); the artist
+      // page is one click further, in that panel.
+      if (probe.platform === 'discord' && probe.discord?.channel_id) {
+        if (document.getElementById('fc-add-panel')) closePanel();
+        else openPostersPanel(btn, probe);
+        return;
+      }
       await openArtist(btn, probe.artist?.slug);
       return;
     }
@@ -158,6 +165,120 @@
     document.getElementById('fc-add-panel')?.remove();
   }
 
+  // ---- Poster picker (#4488) ----
+  // Who posted in this channel lately, read by FabledCurator with its Discord
+  // token, the creator the server is named for (or owned by) ticked. The
+  // source keeps their ids: a username can change on a whim, an id can't, so
+  // the list never quietly stops matching. Names are other people's text —
+  // createElement only.
+
+  function posterSection() {
+    const list = el('div', { class: 'fc-panel__posters' }, [
+      el('div', { class: 'fc-panel__hint', text: 'Reading who posts here…' }),
+    ]);
+    const node = el('div', {}, [
+      el('div', { class: 'fc-panel__label', text: 'Only posts by' }),
+      list,
+    ]);
+    const state = { data: null, picked: new Set() };
+
+    function render() {
+      const posters = withSelected(state.data);
+      if (!posters.length) {
+        list.replaceChildren(el('div', { class: 'fc-panel__hint', text: 'No one has posted here lately.' }));
+        return;
+      }
+      const rows = posters.map((p) => {
+        const id = String(p.id);
+        const box = el('input', { type: 'checkbox', checked: state.picked.has(id) });
+        box.addEventListener('change', () => {
+          if (box.checked) state.picked.add(id);
+          else state.picked.delete(id);
+          hint.textContent = pickHint();
+        });
+        const bits = [`${p.images} image${p.images === 1 ? '' : 's'}`, `${p.messages} message${p.messages === 1 ? '' : 's'}`];
+        if (p.reasons?.length) bits.unshift(p.reasons.join(', '));
+        return el('label', { class: 'fc-panel__radio fc-panel__poster' }, [
+          box,
+          el('span', { text: posterLabel(p) }),
+          el('span', { class: 'fc-panel__poster-meta', text: bits.join(' · ') }),
+        ]);
+      });
+      const hint = el('div', { class: 'fc-panel__hint', text: pickHint() });
+      list.replaceChildren(...rows, hint);
+    }
+
+    function pickHint() {
+      return state.picked.size
+        ? 'Only the ticked people’s posts are taken.'
+        : 'Nothing ticked: everyone’s posts are taken.';
+    }
+
+    async function load() {
+      let r;
+      try {
+        r = await browser.runtime.sendMessage({ type: 'DISCORD_POSTERS', url: window.location.href });
+      } catch (e) {
+        r = { error: e.message };
+      }
+      if (r?.error) {
+        list.replaceChildren(el('div', { class: 'fc-panel__hint', text: `Couldn’t list posters — ${r.error}` }));
+        return;
+      }
+      state.data = r;
+      state.picked = initialPosterPick(r);
+      render();
+    }
+
+    // null until the list has loaded: an add made before then leaves the
+    // source's list as it is instead of clearing it.
+    const request = () => (state.data ? postersRequest(state.data.posters, state.picked) : null);
+    return { node, load, request };
+  }
+
+  function openPostersPanel(btn, probe) {
+    closePanel();
+    const d = probe.discord || {};
+    const posters = posterSection();
+    const saveBtn = el('button', { class: 'fc-panel__btn fc-panel__btn--primary', text: 'Save' });
+    const artistBtn = el('button', { class: 'fc-panel__btn', text: `Open ${probe.artist?.name || 'artist'}` });
+    const cancelBtn = el('button', { class: 'fc-panel__btn', text: 'Close' });
+    const panel = el('div', { id: 'fc-add-panel', class: 'fc-panel' }, [
+      el('div', { class: 'fc-panel__title', text: 'Discord source' }),
+      el('div', { class: 'fc-panel__sub', text: d.channel_id ? channelLabel(d) : serverLabel(d) }),
+      posters.node,
+      el('div', { class: 'fc-panel__actions' }, [artistBtn, cancelBtn, saveBtn]),
+    ]);
+    panel.addEventListener('keydown', (e) => {
+      e.stopPropagation();
+      if (e.key === 'Escape') closePanel();
+    });
+    cancelBtn.addEventListener('click', closePanel);
+    artistBtn.addEventListener('click', () => { closePanel(); openArtist(btn, probe.artist?.slug); });
+    saveBtn.addEventListener('click', async () => {
+      const authors = posters.request();
+      if (!authors) return;
+      saveBtn.disabled = true;
+      try {
+        const r = await browser.runtime.sendMessage({
+          type: 'SET_DISCORD_POSTERS', url: window.location.href, authors,
+        });
+        if (r?.error) {
+          showToast(`Error: ${r.error}`, 'error');
+          return;
+        }
+        showToast(authors.length ? `Only posts by ${authors.map((a) => a.name || a.id).join(', ')}` : 'Taking everyone’s posts', 'success');
+        closePanel();
+      } catch (e) {
+        showToast(`Error: ${e.message}`, 'error');
+      } finally {
+        saveBtn.disabled = false;
+      }
+    });
+    document.body.appendChild(panel);
+    posters.load();
+  }
+
   async function openAddPanel(btn, probe) {
     closePanel();
     const platformName = PLATFORMS[probe.platform]?.name || probe.platform;
@@ -184,6 +305,8 @@
     const d = probe.discord || {};
     const discord = probe.platform === 'discord';
     const choice = panelDefaults(probe, window.location.href);
+    // #4488: on a channel page, pick whose posts the new source takes.
+    const posters = discord && d.channel_id ? posterSection() : null;
 
     const scopeRow = (value, label, disabled) => {
       const input = el('input', {
@@ -234,6 +357,7 @@
       el('div', { class: 'fc-panel__label', text: 'Artist' }),
       el('div', { class: 'fc-panel__combo' }, [nameInput, results]),
       renameRow,
+      ...(posters ? [posters.node] : []),
       hint,
       el('div', { class: 'fc-panel__actions' }, [cancelBtn, addBtn]),
     ]);
@@ -392,6 +516,8 @@
     addBtn.addEventListener('click', async () => {
       const req = addRequest(choice);
       if (!req) return;
+      const authors = posters?.request();
+      if (authors) req.discordAuthors = authors;
       const btn = document.getElementById('fc-add-source-btn');
       addBtn.disabled = true;
       const ok = await add(btn || addBtn, req);
@@ -402,6 +528,7 @@
     document.body.appendChild(panel);
     refresh();
     nameInput.focus();
+    posters?.load();
     nameInput.select();
     // Search what the field opens with — the server's name, usually — so an
     // artist it already matches is picked before the operator types anything.