Add analytics endpoint, price history API, and fix daysAvailable calculation

- Fix daysAvailable to detect gaps >25 days (indicates unit was re-listed)
- Add /unit/:unitCode/price-history endpoint for full price history
- Add /analytics endpoint with monthly trends, seasonality analysis,
  plan performance, historical comparisons, and price per sqft stats
- Add /last-scrape endpoint to return timestamp of most recent data
- Increase priceHistory limit from 5 to 30 for better chart data

server.js

@@ -302,7 +302,6 @@ app.get('/available-units', async (req, res) => {
           area: "$area",
           community: "$community",
           available: "$available",
-          soonest: "$soonest",
           currentPrice: "$currentPrice.price",
           minPrice: "$priceStats.minPrice",
           maxPrice: "$priceStats.maxPrice",