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_psn.json

@@ -1 +1,20 @@
-[]
+[
+  {
+    "hash_id": "8941",
+    "name": "Prodej bytu 3+kk, 102 m² — JITRO",
+    "price": 13994000,
+    "price_formatted": "13 994 000 Kč",
+    "locality": "Litevská 1174/8, Praha 10",
+    "lat": 50.0729,
+    "lon": 14.4767,
+    "disposition": "3+kk",
+    "floor": 2,
+    "area": 102.7,
+    "building_type": "neuvedeno",
+    "ownership": "osobní",
+    "url": "https://psn.cz/prodej/ubytovaci-jednotka-3-kk-litevska-praha-10-vrsovice-lit4219",
+    "source": "psn",
+    "image": "",
+    "scraped_at": "2026-02-25"
+  }
+]