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/app/capabilities/tiktok/scrape/schemas.py

@@ -82,5 +82,6 @@ class ScrapeOutput(BaseModel):
 
     @property
     def billable_units(self) -> int:
-        """One returned item = one billable unit."""
-        return len(self.items)
+        """One returned video = one billable unit; ErrorItems (``errorCode`` set,
+        for blocked/empty targets) are surfaced but never charged."""
+        return sum(1 for item in self.items if not getattr(item, "errorCode", None))

surfsense_backend/app/proprietary/platforms/tiktok/flows/listing.py

@@ -12,9 +12,19 @@ from typing import Any
 
 from ..extraction import parse_video
 from ..extraction.timestamps import now_iso
+from ..schemas import ErrorItem
 from ..targets.types import TikTokTarget
 from . import FetchListingFn
 
+# Profile and search feeds are trust-gated: an anonymous headless session gets an
+# empty body (profile) or no results XHR (search), while hashtag feeds load. We
+# can't tell "genuinely empty" from "blocked" here, so a zero-item listing emits
+# one honest ErrorItem instead of vanishing silently.
+_EMPTY_LISTING_MESSAGE = (
+    "No videos returned. The target may be empty/private/removed, or TikTok "
+    "withheld this feed from anonymous access (common for profiles and search)."
+)
+
 
 async def iter_listing(
     target: TikTokTarget, *, cap: int, fetch_listing: FetchListingFn
@@ -35,3 +45,11 @@ async def iter_listing(
         emitted += 1
         if emitted >= cap:
             return
+    if emitted == 0:
+        yield ErrorItem(
+            url=target.url,
+            input=target.value,
+            error=_EMPTY_LISTING_MESSAGE,
+            errorCode="no_items",
+            scrapedAt=now_iso(),
+        ).to_output()