feat(tiktok): add tiktok.user_search verb for account discovery

Video/general search is login-walled for anonymous sessions, but the Users
tab (/api/search/user) returns public account records without a redirect, so
this exposes the one reliably-unblocked search path. A keyword yields
TikTokProfileItems (name, followers, bio, verification), deduped per query,
capped, and degraded to an ErrorItem when a query is empty/withheld.

Reuses the browser capture (generalized over XHR markers + extractor) and the
shared profile item shape. Billed per account on a new TIKTOK_USER meter
(TIKTOK_MICROS_PER_USER), surfaced on the chat subagent alongside tiktok.scrape.

surfsense_backend/tests/unit/platforms/tiktok/test_user_search.py

@@ -0,0 +1,70 @@
+"""User-search orchestration over a fake fetch (no network).
+
+Drives ``search_tiktok_users``: queries -> captured ``user_info`` -> profile items.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+from app.proprietary.platforms.tiktok import search_tiktok_users
+
+
+def _user(uid: str, unique_id: str, followers: int = 10) -> dict[str, Any]:
+    return {
+        "uid": uid,
+        "unique_id": unique_id,
+        "nickname": unique_id.upper(),
+        "signature": "bio",
+        "follower_count": followers,
+        "total_favorited": 999,
+        "sec_uid": f"sec-{uid}",
+        "enterprise_verify_reason": "official" if uid == "1" else "",
+        "avatar_thumb": {"url_list": [f"https://cdn/{uid}.webp"]},
+    }
+
+
+async def test_user_search_parses_dedupes_and_caps():
+    async def fake_fetch(_url: str, _cap: int) -> list[dict]:
+        return [_user("1", "nasa"), _user("1", "nasa"), _user("2", "nasa2")]
+
+    items = await search_tiktok_users(
+        ["nasa"], per_query=2, fetch_users=fake_fetch
+    )
+
+    assert [i["id"] for i in items] == ["1", "2"]
+    first = items[0]
+    assert first["name"] == "nasa"
+    assert first["nickName"] == "NASA"
+    assert first["profileUrl"] == "https://www.tiktok.com/@nasa"
+    assert first["verified"] is True
+    assert first["fans"] == 10
+    assert first["avatar"] == "https://cdn/1.webp"
+    assert first["secUid"] == "sec-1"
+    assert first["scrapedAt"] is not None
+    assert items[1]["verified"] is False
+
+
+async def test_user_search_empty_query_emits_error_item():
+    async def fake_fetch(_url: str, _cap: int) -> list[dict]:
+        return []
+
+    items = await search_tiktok_users(
+        ["ghost"], per_query=5, fetch_users=fake_fetch
+    )
+
+    assert len(items) == 1
+    assert items[0]["errorCode"] == "no_users"
+    assert items[0]["input"] == "ghost"
+
+
+async def test_user_search_honors_limit_across_queries():
+    async def fake_fetch(_url: str, _cap: int) -> list[dict]:
+        return [_user("1", "a"), _user("2", "b")]
+
+    items = await search_tiktok_users(
+        ["q1", "q2"], per_query=5, limit=3, fetch_users=fake_fetch
+    )
+
+    # 2 from q1 + 1 from q2, then the cross-query limit stops it.
+    assert len(items) == 3