Reliability improvements: retry logic, validation, ratings sync

- Add 3-attempt retry with exponential backoff to Sreality, Realingo,
  Bezrealitky, and PSN scrapers (CityHome and iDNES already had it)
- Add shared validate_listing() in scraper_stats.py; all 6 scrapers now
  validate GPS bounds, price, area, and required fields before output
- Wire ratings to server /api/ratings on page load (merge with
  localStorage) and save (async POST); ratings now persist across
  browsers and devices
- Namespace JS hash IDs as {source}_{id} to prevent rating collisions
  between listings from different portals with the same numeric ID
- Replace manual Czech diacritic table with unicodedata.normalize()
  in merge_and_map.py for correct deduplication of all edge cases
- Correct README schedule docs: every 4 hours, not twice daily

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

scrape_cityhome.py

@@ -14,7 +14,7 @@ import time
 import urllib.request
 from datetime import datetime
 from pathlib import Path
-from scraper_stats import write_stats
+from scraper_stats import write_stats, validate_listing
 
 STATS_FILE = "stats_cityhome.json"
 
@@ -375,6 +375,8 @@ def scrape(max_pages: int | None = None, max_properties: int | None = None):
             "first_seen": _prev_cache.get(f"cityhome_{slug}_{listing['unit_name']}", {}).get("first_seen", datetime.now().strftime("%Y-%m-%d")),
             "last_changed": datetime.now().strftime("%Y-%m-%d") if _prev_cache.get(f"cityhome_{slug}_{listing['unit_name']}", {}).get("price") != price else _prev_cache[f"cityhome_{slug}_{listing['unit_name']}"].get("last_changed", datetime.now().strftime("%Y-%m-%d")),
         }
+        if not validate_listing(result, "cityhome"):
+            continue
         results.append(result)
         properties_fetched += 1