refactor(game): lock-free storage, remove /command, flatten engine wrapper

Three-stage refactor of the game-engine plumbing (game logic untouched):

Stage 1 — lock-free persistence + admin serialisation. Remove the file
lock from repo/fs (the .lock file, the Read/Write-vs-*Safe duality and the
dead ReadSafe polling) and replace the two-step rename with a single atomic
rename so concurrent reads are torn-free without a lock. Serialise the
state-mutating admin writers (init/turn/banish) with one shared router
LimitMiddleware, rewritten to block on the request context instead of a
racy shared 100ms timer.

Stage 2 — remove the obsolete immediate-command path end to end. Players
submit through PUT /api/v1/order; the legacy PUT /api/v1/command path is
deleted across game (route, handler, 24 command factories, Ctrl), backend
(Commands handler/route, engineclient.ExecuteCommands), gateway (dispatch +
executeUserGamesCommand + routing entry), the FlatBuffers/model contract
(UserGamesCommand[Response]) and transcoder, plus every affected
OpenAPI/README/FUNCTIONAL/ARCHITECTURE doc. The integration proxy test is
converted to the order path.

Stage 3 — flatten the REST->engine wrapper. Replace the executor adapter,
the controller package functions and RepoController with one concrete
controller.Service; drop the single-implementation Repo and Storage
interfaces (repo.Repo / fs.FS are now concrete). Handlers depend on a thin
handler.Engine seam and own the domain->REST projection; storage is
resolved once at startup instead of per request.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/internal/engineclient/client.go

@@ -23,7 +23,6 @@ const (
 	pathAdminStatus     = "/api/v1/admin/status"
 	pathAdminTurn       = "/api/v1/admin/turn"
 	pathAdminRaceBanish = "/api/v1/admin/race/banish"
-	pathPlayerCommand   = "/api/v1/command"
 	pathPlayerOrder     = "/api/v1/order"
 	pathPlayerReport    = "/api/v1/report"
 	pathPlayerBattle    = "/api/v1/battle"
@@ -183,16 +182,10 @@ func (c *Client) BanishRace(ctx context.Context, baseURL, raceName string) error
 	}
 }
 
-// ExecuteCommands calls `PUT /api/v1/command` with payload forwarded
+// PutOrders calls `PUT /api/v1/order` with the payload forwarded
 // verbatim. The engine response body is returned verbatim; on 4xx the
-// body is returned alongside ErrEngineValidation so callers can
-// forward the per-command error.
-func (c *Client) ExecuteCommands(ctx context.Context, baseURL string, payload json.RawMessage) (json.RawMessage, error) {
-	return c.forwardPlayerWrite(ctx, baseURL, pathPlayerCommand, payload, "engine command")
-}
-
-// PutOrders calls `PUT /api/v1/order` with the same forwarding
-// semantics as ExecuteCommands.
+// body is returned alongside ErrEngineValidation so callers can forward
+// the per-command error.
 func (c *Client) PutOrders(ctx context.Context, baseURL string, payload json.RawMessage) (json.RawMessage, error) {
 	return c.forwardPlayerWrite(ctx, baseURL, pathPlayerOrder, payload, "engine order")
 }