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>

byty_cityhome.json

@@ -1 +1,42 @@
-[]
+[
+  {
+    "hash_id": "cityhome_na-vaclavce-34_Byt A2.3",
+    "name": "Prodej bytu 3+1, 99 m² — Na Václavce 34",
+    "price": 13490000,
+    "price_formatted": "13 490 000 Kč",
+    "locality": "Na Václavce 34, Praha 5",
+    "lat": 50.0652858,
+    "lon": 14.3931318,
+    "disposition": "3+1",
+    "floor": 2,
+    "area": 99.1,
+    "building_type": "Cihlová",
+    "ownership": "neuvedeno",
+    "url": "https://www.city-home.cz/projekty/na-vaclavce-34/nabidka-nemovitosti/byt-a23",
+    "source": "cityhome",
+    "image": "",
+    "scraped_at": "2026-02-26",
+    "first_seen": "2026-02-26",
+    "last_changed": "2026-02-26"
+  },
+  {
+    "hash_id": "cityhome_na-vaclavce-34_Byt A3.2",
+    "name": "Prodej bytu 3+1, 95 m² — Na Václavce 34",
+    "price": 13490000,
+    "price_formatted": "13 490 000 Kč",
+    "locality": "Na Václavce 34, Praha 5",
+    "lat": 50.0652858,
+    "lon": 14.3931318,
+    "disposition": "3+1",
+    "floor": 3,
+    "area": 95.6,
+    "building_type": "Cihlová",
+    "ownership": "neuvedeno",
+    "url": "https://www.city-home.cz/projekty/na-vaclavce-34/nabidka-nemovitosti/byt-a32",
+    "source": "cityhome",
+    "image": "",
+    "scraped_at": "2026-02-26",
+    "first_seen": "2026-02-26",
+    "last_changed": "2026-02-26"
+  }
+]