Scan updates & robustness

[kernelcoffee]

• Drift detection: a listing page that parses to zero giveaways without SteamGifts' 'No results were found.' marker (pagination--no-results) now raises SteamGiftsScrapeDriftError instead of silently returning an empty list; the sync logs it as a warning in the activity log. Captured a real empty-search page as a sanitized fixture
• Wishlist scans now use max_scan_pages (was hardcoded 1 page) with early stop: an empty or short (<50 rows) page ends any scan, so small lists still cost a single request
• After a complete wishlist scan (end of list actually seen), active giveaways flagged is_wishlist that no longer appear lose the flag (game removed from Steam wishlist); expired rows keep the flag as history, and partial or drift-aborted scans never un-stick anything