feat: surface YouTube + TikTok top comments alongside Reddit (#260)

* feat(normalize): pass YouTube top_comments through with Reddit-compatible shape

_normalize_youtube silently dropped top_comments after enrich_with_comments
populated them, so the downstream signals/render/entity layers never saw
YouTube comments. Map likes->score and text->excerpt so the existing
Reddit-compatible readers Just Work.

Shared _remap_comments helper will be reused for TikTok in a later commit.

* feat(tiktok): fetch top comments via ScrapeCreators when opted in

Mirrors the youtube_comments pattern: new env.is_tiktok_comments_available
gate (requires SCRAPECREATORS_API_KEY + tiktok_comments in INCLUDE_SOURCES),
tiktok.enrich_with_comments ranks posts and fetches via
GET /v1/tiktok/video/comments. Vote field is digg_count; text and user.nickname
come across verbatim. Pipeline calls the enricher right after TikTok search
when the gate is open.

Comment-fetch errors never crash the pipeline — the enricher returns an
empty list on 4xx/5xx.

* feat(normalize): pass TikTok top_comments through with digg_count->score mapping

Instagram uses the same shortform normalizer and has no comment fetcher
today, so the key is harmlessly absent there — no Instagram regression.

* feat(signals): add YouTube + TikTok top-comment score to engagement formula

Mirrors Reddit's 10% top-comment slot. Without top_comments present, the
formula reduces to views-dominant weighting; with a high-signal comment,
the item gets a meaningful bump (log1p(10k) ~ 9.2, weighted 0.10 = ~0.92
on the engagement score).

Updated the existing dominant-weight and missing-fields tests to the new
weights (0.45/0.32/0.13 for YT, 0.45/0.27/0.18 for TT). Views still dominate.

* feat(render): source-aware thresholds and vote labels for top comments

10 upvotes on Reddit signals community interest; 10 likes on a viral
TikTok is noise. Introduce per-source minimums (reddit 10, youtube 50,
tiktok 500) and native vote labels ('upvotes' for Reddit, 'likes' for
YT/TT). First-pass numbers — tune after live observation.

* docs: generalize top-comment quoting to YouTube + TikTok, add tiktok_comments opt-in

Synthesis instructions previously called out Reddit top comments only.
Now cover Reddit/YouTube/TikTok uniformly with source-appropriate vote
labels (upvotes vs likes), and explicitly frame YT transcript highlights
and comments as complementary signals. README and setup-wizard copy
document the new tiktok_comments INCLUDE_SOURCES token.

---------

Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>

scripts/lib/pipeline.py

@@ -887,7 +887,11 @@ def _retrieve_stream(
             hashtags=tiktok_hashtags,
             creators=tiktok_creators,
         )
-        return tiktok.parse_tiktok_response(result), {}
+        items = tiktok.parse_tiktok_response(result)
+        if items and env.is_tiktok_comments_available(config):
+            sc_token = config.get("SCRAPECREATORS_API_KEY", "")
+            tiktok.enrich_with_comments(items, token=sc_token)
+        return items, {}
     if source == "instagram":
         # Use raw_topic so expand_instagram_queries() generates diverse variants
         # from the original user topic, not the planner's narrowed search_query.