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_psn.py

@@ -12,6 +12,7 @@ import logging
 import re
 import subprocess
 import time
+from datetime import datetime
 from pathlib import Path
 
 logger = logging.getLogger(__name__)
@@ -109,7 +110,20 @@ def format_price(price: int) -> str:
     return " ".join(reversed(parts)) + " Kč"
 
 
+def load_previous(json_path: str = "byty_psn.json") -> dict[str, str]:
+    """Load first_seen dates from previous run, keyed by hash_id."""
+    path = Path(json_path)
+    if not path.exists():
+        return {}
+    try:
+        data = json.loads(path.read_text(encoding="utf-8"))
+        return {str(e["hash_id"]): e.get("first_seen", "") for e in data if "hash_id" in e}
+    except (json.JSONDecodeError, KeyError):
+        return {}
+
+
 def scrape(max_pages: int | None = None, max_properties: int | None = None):
+    previous_first_seen = load_previous()
     logger.info("=" * 60)
     logger.info("Stahuji inzeráty z PSN.cz")
     logger.info(f"Cena: do {format_price(MAX_PRICE)}")
@@ -282,8 +296,12 @@ def scrape(max_pages: int | None = None, max_properties: int | None = None):
         project_slug = unit.get("_project_slug", "")
         detail_url = f"{BASE_URL}/projekt/{project_slug}/{slug}" if slug else f"{BASE_URL}/projekt/{project_slug}"
 
+        today = datetime.now().strftime("%Y-%m-%d")
+        hash_id = unit.get("id", slug)
+        first_seen = previous_first_seen.get(str(hash_id), "") or today
+
         result = {
-            "hash_id": unit.get("id", slug),
+            "hash_id": hash_id,
             "name": f"Prodej bytu {disp} {area} m² — {unit.get('_project_name', '')}",
             "price": int(price),
             "price_formatted": format_price(int(price)),
@@ -298,6 +316,8 @@ def scrape(max_pages: int | None = None, max_properties: int | None = None):
             "url": detail_url,
             "source": "psn",
             "image": "",
+            "first_seen": first_seen,
+            "last_updated": today,
         }
         results.append(result)
         properties_fetched += 1