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>

scraper_stats.py

@@ -1,13 +1,53 @@
-"""Shared utility for writing per-scraper run statistics to JSON."""
+"""Shared utilities for scraper run statistics and listing validation."""
 from __future__ import annotations
 
 import json
+import logging
 import os
 from pathlib import Path
 
 HERE = Path(__file__).parent
 DATA_DIR = Path(os.environ.get("DATA_DIR", HERE))
 
+_val_log = logging.getLogger(__name__)
+
+_REQUIRED_FIELDS = ("hash_id", "price", "locality", "lat", "lon", "url", "source")
+
+
+def validate_listing(listing: dict, context: str = "") -> bool:
+    """
+    Validate a listing dict before it is written to the output JSON.
+    Returns True if valid, False if the listing should be skipped.
+    Logs a warning for each invalid listing.
+    """
+    prefix = f"[{context}] " if context else ""
+
+    for field in _REQUIRED_FIELDS:
+        val = listing.get(field)
+        if val is None or val == "":
+            _val_log.warning(f"{prefix}Skipping listing — missing field '{field}': {listing.get('hash_id', '?')}")
+            return False
+
+    price = listing.get("price")
+    if not isinstance(price, (int, float)) or price <= 0:
+        _val_log.warning(f"{prefix}Skipping listing — invalid price={price!r}: {listing.get('hash_id', '?')}")
+        return False
+
+    lat, lon = listing.get("lat"), listing.get("lon")
+    if not isinstance(lat, (int, float)) or not isinstance(lon, (int, float)):
+        _val_log.warning(f"{prefix}Skipping listing — non-numeric GPS lat={lat!r} lon={lon!r}: {listing.get('hash_id', '?')}")
+        return False
+    if not (47.0 <= lat <= 52.0) or not (12.0 <= lon <= 19.0):
+        _val_log.warning(f"{prefix}Skipping listing — GPS outside Czech Republic lat={lat} lon={lon}: {listing.get('hash_id', '?')}")
+        return False
+
+    area = listing.get("area")
+    if area is not None and (not isinstance(area, (int, float)) or area <= 0):
+        _val_log.warning(f"{prefix}Skipping listing — invalid area={area!r}: {listing.get('hash_id', '?')}")
+        return False
+
+    return True
+
 
 def write_stats(filename: str, stats: dict) -> None:
     """Write scraper run stats dict to the data directory."""