Add first_seen/last_updated timestamps to track property freshness

Each property record now carries two date fields:
- first_seen: date the listing first appeared (preserved across runs)
- last_updated: date of the most recent scrape that included it

All 6 scrapers (Sreality, Realingo, Bezrealitky, iDNES, PSN, CityHome)
set these fields during scraping. Cached results preserve first_seen and
refresh last_updated. PSN and CityHome gain a load_previous() helper to
track first_seen across runs (they lacked caching before).

The merge script keeps the earliest first_seen and latest last_updated
when deduplicating listings across sources.

The HTML map now shows dates in popups ("Přidáno: DD.MM.YYYY"), displays
a green "NOVÉ" badge on newly discovered listings, and adds a "Přidáno"
dropdown filter (24h / 3 days / 7 days / 14 days) for spotting new ones.

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

scrape_bezrealitky.py

@@ -13,6 +13,7 @@ import math
 import re
 import time
 import urllib.request
+from datetime import datetime
 from pathlib import Path
 
 logger = logging.getLogger(__name__)
@@ -284,10 +285,14 @@ def scrape(max_pages: int | None = None, max_properties: int | None = None):
         # Check cache — if hash_id exists and price unchanged, reuse
         adv_id = int(adv["id"])
         adv_price = adv.get("price", 0) or 0
+        today = datetime.now().strftime("%Y-%m-%d")
         cached = cache.get(adv_id)
         if cached and cached.get("price") == adv_price:
             cache_hits += 1
             logger.debug(f"Cache hit for id={adv_id}")
+            cached["last_updated"] = today
+            if "first_seen" not in cached:
+                cached["first_seen"] = today
             results.append(cached)
             continue
 
@@ -339,6 +344,11 @@ def scrape(max_pages: int | None = None, max_properties: int | None = None):
         if not address:
             address = adv.get('address({"locale":"CS"})', "Praha")
 
+        # Preserve first_seen from cache if this is a price-changed re-fetch
+        first_seen = today
+        if cached and "first_seen" in cached:
+            first_seen = cached["first_seen"]
+
         result = {
             "hash_id": int(adv["id"]),
             "name": f"Prodej bytu {DISPOSITION_LABELS.get(disp, '?')} {adv.get('surface', '?')} m²",
@@ -355,6 +365,8 @@ def scrape(max_pages: int | None = None, max_properties: int | None = None):
             "url": f"{BASE_URL}/nemovitosti-byty-domy/{uri}",
             "source": "bezrealitky",
             "image": "",
+            "first_seen": first_seen,
+            "last_updated": today,
         }
         results.append(result)
         properties_fetched += 1