feat(tiktok): emit graceful ErrorItem for blocked/empty listings

Profile and search feeds are trust-gated: an anonymous headless session
gets an empty item_list (profile) or no results XHR (search), while
hashtag feeds load. A zero-item listing now yields one honest ErrorItem
(errorCode="no_items") instead of vanishing silently, and ErrorItems are
excluded from billing so a blocked target is surfaced but never charged.

surfsense_backend/tests/unit/capabilities/tiktok/scrape/test_schemas.py

@@ -9,6 +9,7 @@ from app.capabilities.tiktok.scrape.schemas import (
     MAX_TIKTOK_ITEMS,
     MAX_TIKTOK_SOURCES,
     ScrapeInput,
+    ScrapeOutput,
 )
 
 pytestmark = pytest.mark.unit
@@ -43,3 +44,15 @@ def test_rejects_more_sources_than_the_cap():
     too_many = [f"tag{i}" for i in range(MAX_TIKTOK_SOURCES + 1)]
     with pytest.raises(ValidationError):
         ScrapeInput(hashtags=too_many)
+
+
+def test_error_items_are_not_billed():
+    # Real videos count; ErrorItems (blocked/empty targets) are surfaced free.
+    out = ScrapeOutput(
+        items=[
+            {"id": "1", "webVideoUrl": "https://tiktok.com/@a/video/1"},
+            {"errorCode": "no_items", "input": "nasa", "error": "blocked"},
+        ]
+    )
+    assert len(out.items) == 2
+    assert out.billable_units == 1