feat: INCLUDE_SOURCES config + TikTok/Instagram opt-in in NUX

- INCLUDE_SOURCES=tiktok,instagram in .env forces sources on for all
  query types, bypassing the tier system
- NUX shows opt-in modal after ScrapeCreators key is saved: "Also
  search TikTok and Instagram?" with honest call-usage warning
- Tier system preserved as default — override only when INCLUDE_SOURCES set

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/lib/env.py

@@ -374,6 +374,7 @@ def get_config() -> Dict[str, Any]:
         ('TRUTHSOCIAL_TOKEN', None),
         ('FROM_BROWSER', None),
         ('SETUP_COMPLETE', None),
+        ('INCLUDE_SOURCES', None),
     ]
 
     for key, default in keys: