fix(bikeshare): re-derive a station's missing capacity on every poll

A station whose feed publishes no capacity froze its capacity at the
first status poll's bikes + docks, or at the placeholder 15 when that
poll had no row for it, so later cards read "10 avail · 20 docks · 15
cap" and coloured by the wrong ratio. Each poll now resolves it afresh.

docs/CURRENT-STATE.md

@@ -1779,6 +1779,8 @@ limit, as well as when a tile decodes or the source resets.
 GBFS null or empty availability counts retain an unknown value through station
 parsing and rendering; they use the neutral station color rather than reporting
 an empty dock. Explicit zero and caller-supplied numeric fallbacks remain valid.
+A station whose `station_information` publishes no capacity takes bikes plus
+docks from each status poll (15 until its status arrives), not from the first.
 
 ## Terrain height cache bound