Track first_seen/last_changed per property, add map filters and clickable legend

Scraper changes (all 6 sources):
- Add first_seen: date the hash_id was first scraped, never overwritten
- Add last_changed: date the price last changed (= first_seen when new)
- PSN and CityHome load previous output as a lightweight cache to compute these fields
- merge_and_map.py preserves earliest first_seen when deduplicating cross-source duplicates

Map popup:
- Show "Přidáno: YYYY-MM-DD" and "Změněno: YYYY-MM-DD" in each property popup
- NOVÉ badge and pulsing marker now driven by first_seen == today (more accurate than scraped_at)

Map filters (sidebar):
- New "Přidáno / změněno" dropdown: 1, 2, 3, 4, 5, 7, 14, 30 days or all
- Clickable price/m² legend bands: click to filter to that band, multi-select supported
- "✕ Zobrazit všechny ceny" reset link appears when any band is active

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

merge_and_map.py

@@ -79,6 +79,10 @@ def main():
         if key in seen_keys:
             dupes += 1
             existing = seen_keys[key]
+            # Preserve earliest first_seen across sources
+            dup_fs = e.get("first_seen", "")
+            if dup_fs and (not existing.get("first_seen") or dup_fs < existing["first_seen"]):
+                existing["first_seen"] = dup_fs
             # Log it
             print(f"  Duplikát: {e['locality']} | {format_price(e['price'])} | {e.get('area', '?')} m² "
                   f"({e.get('source', '?')} vs {existing.get('source', '?')})")