feat: add a recent satellite imagery tool

Add get_recent_imagery. It searches the recent Harmonized Landsat and
Sentinel-2 catalog for an area, picks the image the layer would show
with the layer's own ranking, and returns it with its date, sensor and
cloud cover, falling back to the VIIRS daily overview.

docs/CURRENT-STATE.md

@@ -14,7 +14,8 @@ by identifier, and tracks), satellites (next
 pass over a point, and those overhead now), public cameras (including a
 camera's current image), radio stations, place search, routing, bike-share
 stations, transit vehicles, road traffic flow, weather, weather map images
-(radar, satellite, lightning), wind, regional briefs, tropical cyclones, fire
+(radar, satellite, lightning), wind, the most recent satellite image of an
+area, regional briefs, tropical cyclones, fire
 perimeters, terrain height, military installations and map features, plus a
 combined situation brief, the app's heads-up display caption, and a link that
 opens the app over an area. Tools reuse the layers' portable source factories, take a