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>

README.md

@@ -151,7 +151,7 @@ The project includes a Docker setup for unattended operation with a cron-based s
 │  PID 1: python3 -m http.server :8080    │
 │         serves /app/data/               │
 │                                         │
-│  crond:  runs run_all.sh at 06:00/18:00 │
+│  crond:  runs run_all.sh every 4 hours  │
 │          Europe/Prague timezone          │
 │                                         │
 │  /app/        -- scripts (.py, .sh)     │
@@ -160,7 +160,7 @@ The project includes a Docker setup for unattended operation with a cron-based s
 └─────────────────────────────────────────┘
 ```
 
-On startup, the HTTP server starts immediately. The initial scrape runs in the background. Subsequent cron runs update data in-place twice daily at 06:00 and 18:00 CET/CEST.
+On startup, the HTTP server starts immediately. The initial scrape runs in the background. Subsequent cron runs update data in-place every 4 hours.
 
 ### Quick start
 
@@ -208,7 +208,7 @@ Validation targets run scrapers with `--max-pages 1 --max-properties 10` for a f
 ├── build/
 │   ├── Dockerfile          # Container image definition (python:3.13-alpine)
 │   ├── entrypoint.sh       # Container entrypoint (HTTP server + cron + initial scrape)
-│   ├── crontab             # Cron schedule (06:00 and 18:00 CET)
+│   ├── crontab             # Cron schedule (every 4 hours)
 │   └── CONTAINER.md        # Container-specific documentation
 └── .gitignore              # Ignores byty_*.json, __pycache__, .vscode
 ```