feat: add ship tools

Add vessels_in_area, find_vessel and get_vessel_track over the live
vessel source, and a ships section in situation_brief. Without an AIS
key the source's own message reaches clients as an unavailable feed.

docs/TOOLS.md

@@ -99,6 +99,9 @@ public feeds the sources already use.
 | `find_aircraft`       | `aircraft`    | Aircraft anywhere by callsign, ICAO address or registration    |
 | `get_aircraft_track`  | `aircraft`    | Recent positions of one aircraft, thinned to 200 points        |
 | `get_aircraft_info`   | `aircraft`    | Aircraft type and registration, and flight route, from adsbdb  |
+| `vessels_in_area` | `vessels` | Ships reported by AIS in an area, nearest first, optionally by type |
+| `find_vessel` | `vessels` | Ships anywhere by MMSI, IMO number or name |
+| `get_vessel_track` | `vessels` | Recent positions of one ship, thinned to 200 points |
 | `next_satellite_pass` | `satellites` | Next pass over a point (default the ISS), with naked-eye visibility |
 | `satellites_overhead` | `satellites` | Satellites in a CelesTrak group above a point now, highest first |
 | `find_cctv_cameras` | `cctv` | Public cameras in an area, nearest first |
@@ -116,5 +119,5 @@ public feeds the sources already use.
 | `get_terrain_height` | `terrain` | Ground, geoid and ellipsoid heights at up to 20 points |
 | `find_military_installations` | `installations` | OpenStreetMap military sites in an area of at most 10° per side |
 | `get_map_features` | `features` | Administrative areas, named places or monuments at a location (needs Overpass) |
-| `situation_brief` | `weather` | Weather, earthquakes, fires, aircraft and cyclones for an area, by section |
+| `situation_brief` | `weather` | Weather, earthquakes, fires, aircraft, ships and cyclones for an area, by section |
 | `get_hud_caption` | `weather`, `summary` | The app's heads-up display caption for an area |