feat(game): add GET /{gameId}/fen-history endpoint

Returns a FEN string for every ply of the game (initial position + one
per move) by replaying moves via ruleSet.applyMove and exporting each
GameContext to FEN. Enables full per-move engine analysis from clients
without requiring a chess library on their side.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

modules/api/src/main/scala/de/nowchess/api/dto/FenHistoryDto.scala

@@ -0,0 +1,3 @@
+package de.nowchess.api.dto
+
+final case class FenHistoryDto(fens: List[String])