feat(tiktok): add TikTok as 7th signal source via Apify

Add TikTok search, scoring, and rendering using the Apify platform
(clockworks/tiktok-scraper actor). Users bring their own APIFY_API_TOKEN
($5/month free credits, no CC required). The shared apify_client_wrapper
module is designed for reuse by future Facebook/Instagram sources.

- New modules: tiktok.py (search + caption extraction), apify_client_wrapper.py
- Schema: TikTokItem dataclass, shares field on Engagement, Report.tiktok
- Pipeline: normalize → filter → score → sort → dedupe → cross-link → render
- Scoring: 0.50*log1p(views) + 0.30*log1p(likes) + 0.20*log1p(comments)
- SKILL.md bumped to v2.7 with TikTok stats, citations, and security docs
- 26 unit tests covering relevance, normalize, score, dedupe, render, round-trip

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/lib/env.py

@@ -203,6 +203,7 @@ def get_config() -> Dict[str, Any]:
         ('OPENAI_MODEL_PIN', None),
         ('XAI_MODEL_POLICY', 'latest'),
         ('XAI_MODEL_PIN', None),
+        ('APIFY_API_TOKEN', None),
         ('AUTH_TOKEN', None),
         ('CT0', None),
     ]
@@ -406,6 +407,15 @@ def is_polymarket_available() -> bool:
     return True
 
 
+def is_apify_available(config: Dict[str, Any]) -> bool:
+    """Check if Apify token is configured for TikTok/social scraping.
+
+    Returns True if APIFY_API_TOKEN is set. One token covers
+    TikTok, Facebook, Instagram (all Apify-backed sources).
+    """
+    return bool(config.get('APIFY_API_TOKEN'))
+
+
 def get_x_source_status(config: Dict[str, Any]) -> Dict[str, Any]:
     """Get detailed X source status for UI decisions.