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>

tests/test_discord_client.py

@@ -424,3 +424,60 @@ def test_the_adapter_authenticates_with_the_token_and_keys_by_identity(tmp_path)
         _msg(9, attachments=[{"id": "300", "url": "https://cdn/a/1.png"}])
     )
     assert _ledger_key(media) == "9:300"
+
+
+# -- the poster picker (#4488) -------------------------------------------------
+
+def _person(pid, username, global_name=None, **extra):
+    return {"id": pid, "username": username, "global_name": global_name,
+            "messages": 1, "images": 0, **extra}
+
+
+def test_the_creator_a_server_is_named_for_is_suggested():
+    ranked = dc.rank_posters("Todding's Server", "99", [
+        _person("8", "jakeboii", "Jake Boii", images=3),
+        _person("7", "todding", "Todding", images=1),
+    ])
+    assert ranked[0]["id"] == "7" and ranked[0]["suggested"] is True
+    assert ranked[0]["reasons"] == ["name matches the server"]
+    assert ranked[1]["suggested"] is False
+
+
+def test_owning_the_server_and_matching_its_name_outranks_either_alone():
+    ranked = dc.rank_posters("The Official Todding Discord", "7", [
+        _person("5", "toddingfan"),
+        _person("7", "t0dd", "Todding"),
+    ])
+    assert [r["id"] for r in ranked if r["suggested"]] == ["7"]
+    assert ranked[0]["reasons"] == ["owns the server", "name matches the server"]
+
+
+def test_no_signal_suggests_no_one_and_bots_never():
+    assert not any(r["suggested"] for r in dc.rank_posters("Art Club", None, [
+        _person("1", "alice"), _person("2", "bob"),
+    ]))
+    ranked = dc.rank_posters("Todding's Server", None, [_person("3", "toddingbot", bot=True)])
+    assert ranked[0]["suggested"] is False
+
+
+def test_recent_posters_tallies_a_shallow_window_by_id():
+    creator = {"id": "7", "username": "todding", "global_name": "Todding"}
+    other = {"id": "8", "username": "jakeboii", "global_name": "Jake Boii"}
+    art = [{"url": "https://cdn/a/1.png"}]
+    page = [_msg(i, author=creator, attachments=art) for i in range(300, 240, -1)]
+    page += [_msg(i, author=other) for i in range(240, 200, -1)]
+    routes = {
+        "/guilds/1": _ok({"id": "1", "name": "Todding's Server", "owner_id": "7"}),
+        ("/channels/2/messages", None): _ok(page),
+        ("/channels/2/messages", "201"): _ok([_msg(150, author=other)]),
+    }
+    client = _client(routes)
+    got = client.recent_posters("1", "2", max_messages=150)
+    assert got["server"] == "Todding's Server" and got["owner_id"] == "7"
+    assert got["scanned"] == 101
+    first = got["posters"][0]
+    assert (first["id"], first["messages"], first["images"], first["suggested"]) == ("7", 60, 60, True)
+    assert got["posters"][1]["messages"] == 41
+    # It asked for no more than the window: 100, then the 50 left of 150.
+    limits = [p.get("limit") for e, p in client._session.calls if e.endswith("/messages")]
+    assert limits == [100, 50]