Add US Mortgage Calculator to Finance

Free mortgage calculation API: no authentication, no key, no rate limit
and no signup. HTTPS and CORS confirmed against the live endpoint from a
third-party origin.

Endpoints cover monthly payment and amortization, affordability,
refinance break-even, closing costs, PMI and MIP, and property tax
reference data for all fifty states. The state data is also published
under CC BY 4.0.

Checked before opening: no mortgage or amortization API is currently
listed, the entry is alphabetical between Twelve Data and VAT
Validation, and the description is 76 characters.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011WY6ZEBhp1yhG6ktdHHEv3

README.md

@@ -930,6 +930,7 @@ API | Description | Auth | HTTPS | CORS |
 | [Top 5 Stocks](https://top5stocks.netlify.app/developers) | Daily AI-ranked stock and crypto watchlists | No | Yes | Unknown | |
 | [Tradier](https://developer.tradier.com) | US equity/option market data (delayed, intraday, historical) | `OAuth` | Yes | Yes | |
 | [Twelve Data](https://twelvedata.com/) | Stock market data (real-time & historical) | `apiKey` | Yes | Unknown | |
+| [US Mortgage Calculator](https://www.usmortgagecalc.com/developers/api) | Mortgage payment, amortization, affordability and 50-state property tax data | No | Yes | Yes | |
 | [VAT Validation](https://www.abstractapi.com/vat-validation-rates-api) | Validate VAT numbers and calculate VAT rates | `apiKey` | Yes | Yes | |
 | [WallstreetBets](https://dashboard.nbshare.io/apps/reddit/api/) | WallstreetBets Stock Comments Sentiment Analysis | No | Yes | Unknown | |
 | [Yahoo Finance](https://www.yahoofinanceapi.com/) | Real time low latency Yahoo Finance API for stock market, crypto currencies, and currency exchange | `apiKey` | Yes | Yes | |