Add global request delay floor + per-source delay management.
- 054: seed 'global'=0 + per-source rows (avito=7, cian=5, n1=5, domrf=5, rosreestr=5)
using ON CONFLICT DO NOTHING; reuses existing 053 table from #484
- scraper_settings.py: get_scraper_delay() = max(per_source, global); _GLOBAL_KEY,
refactored _get_setting_cached(); preserved yandex umbrella alias logic
- admin.py: GET /scraper-settings + PUT /scraper-settings/{source} (ge=0.0, le=60.0);
cache invalidation on update; CAST(:d AS numeric) per psycopg v3
- avito/cian/n1 scrapers wired (yandex was wired by #484)
- 18 unit tests pass (10 original updated for max() + 8 new global delay tests)
Closes follow-up gap from #486 (Cian admin slider min=0 now accepted by API).
