feat(weather): global rain radar from RainViewer

Add a Global (RainViewer) option to Rain radar beside the US MRMS
radar. The proxy pins RainViewer's tile host, validates frames and
Web Mercator tile coordinates, and limits upstream tile requests to
80 a minute. Globe basemaps draw the tiles; on 3D Tiles the shell
composes them into one equirectangular image per frame and detail
window.

Co-authored-by: Sandiv Dathunarachchi <[email protected]>

DATA_SOURCES.md

@@ -18,6 +18,7 @@ How to read this:
 | **NOAA GFS (wind)** (`noaa-gfs-bdp-pds.s3.amazonaws.com`) | Global 10 m wind, optional 2 m temperature and mean sea-level pressure for Wind. Keyless; latest 6-hourly 0.25° cycle, byte-range GRIB2 reads, cached for an hour | U.S. public domain (NOAA); keyless via NOAA Open Data on AWS | "NOAA Global Forecast System (GFS)" (courtesy; not an endorsement) |
 | **ECMWF IFS (wind)** (`data.ecmwf.int/forecasts`, ECMWF Open Data) | The alternative Wind model: 10 m wind, optional 2 m temperature and mean sea-level pressure. Keyless; latest 6-hourly 0.25° run, byte-range GRIB2 reads, cached for an hour | [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/) plus the [ECMWF Terms of Use](https://apps.ecmwf.int/datasets/licences/general/) | "This service is based on data and products of the European Centre for Medium-Range Weather Forecasts (ECMWF)", with the CC BY 4.0 link, the modification notice and ECMWF's liability disclaimer — shown in-app |
 | **NOAA nowCOAST observed weather** (`nowcoast.noaa.gov/geoserver/observations/{weather_radar,satellite}/ows`, WMS 1.1.1) | Rain radar (MRMS base reflectivity, contiguous US, about 4-minute updates) and Satellite clouds (GOES-19/18 Band 14 regional infrared, about 5-minute updates; global infrared mosaic, hourly). Keyless; layer metadata refreshed every 2 minutes, exact-time images cached for up to 24 hours | [NOAA disclaimer](https://oceanservice.noaa.gov/disclaimer.html) | "NOAA nowCOAST · NWS/OAR MRMS radar; NESDIS GOES and global satellite partners" |
+| **RainViewer** (`api.rainviewer.com`, `tilecache.rainviewer.com`) | Rain radar, Global: past 2 hours of radar composite tiles at 10-minute steps, zoom ≤ 7. Keyless; manifest cached 5 minutes, tiles cached like other weather tiles, upstream limited to 80 tile requests a minute | [RainViewer API terms](https://www.rainviewer.com/api.html) | RainViewer (rainviewer.com), linked in-app |
 | **NOAA nowCOAST lightning density** (`nowcoast.noaa.gov/geoserver/observations/lightning_detection/ows`) | Lightning density: 15-minute accumulated strike density on an approximately 8 km grid, Americas and Pacific. Keyless; layer metadata refreshed every 10 minutes, exact-time images cached for up to 24 hours | NOAA Level-5 derived product from Vaisala NLDN/GLD360; the [product description](https://ocean.weather.gov/lightning/lightning_pdd.php) permits public distribution of this density product, not of raw Vaisala detections | "NOAA/NWS nowCOAST; derived from Vaisala NLDN/GLD360" |
 | **NOAA NHC / CPHC advisories** (`www.nhc.noaa.gov/CurrentStorms.json` + the `mapservices.weather.noaa.gov` tropical weather summary MapServer) | Cyclone advisories: storm positions, forecast tracks, points and cones for the Atlantic and eastern/central North Pacific. Keyless through `/api/cyclones`; cached for 5 minutes | [NWS public-data terms](https://www.weather.gov/disclaimer); no endorsement implied | "NOAA/NWS NHC / CPHC" |
 | **OpenStreetMap** | Map and place data (roads, military areas and names, ALPR, place search, outlines); via vector tiles, hourly ALPR extract, bundled data, Nominatim and optional self-configured Overpass | [ODbL 1.0](https://opendatacommons.org/licenses/odbl/1-0/) | © OpenStreetMap contributors |