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>

scrape_bezrealitky.py

@@ -362,6 +362,8 @@ def scrape(max_pages: int | None = None, max_properties: int | None = None):
             "source": "bezrealitky",
             "image": "",
             "scraped_at": datetime.now().strftime("%Y-%m-%d"),
+            "first_seen": cached.get("first_seen", datetime.now().strftime("%Y-%m-%d")) if cached else datetime.now().strftime("%Y-%m-%d"),
+            "last_changed": datetime.now().strftime("%Y-%m-%d"),
         }
         results.append(result)
         properties_fetched += 1