feat(tiktok): add tiktok.comments verb for public comment scraping

Comments load over a signed /api/comment/list XHR that TikTok serves to
anonymous sessions once the comments panel is opened (unlike profile-video and
general-search feeds), so this is a reliable verb. Given video URLs it returns
CommentItems (text, author, likes, reply counts; replies carry repliesToId),
deduped per video, capped, and degraded to an ErrorItem for empty/withheld
videos or a bad_url ErrorItem for non-video inputs.

Generalizes the browser capture over a pluggable interaction step so the
comments flow (open panel, scroll the panel to paginate) reuses the same
warm+capture scaffolding as listing/user-search. Billed per comment on a new
TIKTOK_COMMENT meter (TIKTOK_MICROS_PER_COMMENT, matching the per-comment
market), surfaced on the chat subagent alongside tiktok.scrape/user_search.

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

@@ -0,0 +1,89 @@
+"""Comments orchestration over a fake fetch (no network).
+
+Drives ``scrape_tiktok_comments``: video URLs -> captured raw comments -> items.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+from app.proprietary.platforms.tiktok import scrape_tiktok_comments
+
+_VIDEO = "https://www.tiktok.com/@bob/video/123"
+
+
+def _comment(cid: str, reply_id: str = "0") -> dict[str, Any]:
+    return {
+        "cid": cid,
+        "text": f"comment {cid}",
+        "digg_count": 7,
+        "reply_comment_total": 2,
+        "create_time": 1700000000,
+        "reply_id": reply_id,
+        "user": {
+            "uid": "u1",
+            "unique_id": "alice",
+            "nickname": "Alice",
+            "avatar_thumb": {"url_list": ["https://cdn/a.webp"]},
+        },
+    }
+
+
+async def test_comments_parse_dedupe_and_cap():
+    async def fake_fetch(_url: str, _cap: int) -> list[dict]:
+        return [_comment("1"), _comment("1"), _comment("2", reply_id="1")]
+
+    items = await scrape_tiktok_comments(
+        [_VIDEO], per_video=2, fetch_comments_fn=fake_fetch
+    )
+
+    assert [i["id"] for i in items] == ["1", "2"]
+    first = items[0]
+    assert first["text"] == "comment 1"
+    assert first["videoWebUrl"] == _VIDEO
+    assert first["diggCount"] == 7
+    assert first["uniqueId"] == "alice"
+    assert first["avatar"] == "https://cdn/a.webp"
+    assert first["createTimeISO"] is not None
+    assert first["repliesToId"] is None  # reply_id "0" == top-level
+    assert first["scrapedAt"] is not None
+    assert items[1]["repliesToId"] == "1"  # a reply carries its parent id
+
+
+async def test_empty_video_emits_error_item():
+    async def fake_fetch(_url: str, _cap: int) -> list[dict]:
+        return []
+
+    items = await scrape_tiktok_comments(
+        [_VIDEO], per_video=5, fetch_comments_fn=fake_fetch
+    )
+
+    assert len(items) == 1
+    assert items[0]["errorCode"] == "no_comments"
+    assert items[0]["input"] == "123"
+
+
+async def test_non_video_url_emits_bad_url_error():
+    async def fake_fetch(_url: str, _cap: int) -> list[dict]:
+        raise AssertionError("should not fetch for a non-video URL")
+
+    items = await scrape_tiktok_comments(
+        ["https://www.tiktok.com/@bob"], per_video=5, fetch_comments_fn=fake_fetch
+    )
+
+    assert len(items) == 1
+    assert items[0]["errorCode"] == "bad_url"
+
+
+async def test_comments_honor_limit_across_videos():
+    async def fake_fetch(_url: str, _cap: int) -> list[dict]:
+        return [_comment("1"), _comment("2")]
+
+    items = await scrape_tiktok_comments(
+        [_VIDEO, "https://www.tiktok.com/@bob/video/456"],
+        per_video=5,
+        limit=3,
+        fetch_comments_fn=fake_fetch,
+    )
+
+    assert len(items) == 3