feat(transit): add Lisbon's Carris Metropolitana buses

Keyless GTFS-Realtime feed from Carris Metropolitana, documented in the
operator's public API repository. No licence is stated for the realtime
endpoint (the static GTFS is CC BY 4.0), so it is credited as a courtesy on
the same basis as the Metro Transit and OVapi feeds. The feed publishes km/h,
so a new `speedUnit` registry field lets the proxy convert to m/s. Route ids
wrap the GTFS route id in an area prefix and are read as the line number.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

changelog.d/szczecin-lisbon-transit.md

@@ -0,0 +1,6 @@
+- Add Szczecin, Poland and the Lisbon Metropolitan Area to the Transit layer.
+  Szczecin's buses and trams come from ZDiTM's keyless feed under CC0 1.0; it
+  carries no route ids, so vehicles show as buses. Lisbon's buses come from
+  Carris Metropolitana's keyless feed, which states no licence of its own and
+  is credited as a courtesy like Metro Transit and OVapi; its km/h speeds are
+  converted to m/s and its route ids read as the line number (IMBrandao).

changelog.d/szczecin-transit.md

@@ -1,3 +0,0 @@
-- Add Szczecin, Poland to the Transit layer: buses and trams from ZDiTM's
-  keyless GTFS-Realtime feed, published under CC0 1.0. The feed carries no
-  route ids, so vehicles show as buses (IMBrandao).