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/openapi.yaml

@@ -981,37 +981,6 @@ paths:
           $ref: "#/components/responses/NotImplementedError"
         "500":
           $ref: "#/components/responses/InternalError"
-  /api/v1/user/games/{game_id}/commands:
-    post:
-      tags: [User]
-      operationId: userGamesCommands
-      summary: Forward an engine command batch
-      security:
-        - UserHeader: []
-      parameters:
-        - $ref: "#/components/parameters/XUserID"
-        - $ref: "#/components/parameters/GameID"
-      requestBody:
-        required: true
-        content:
-          application/json:
-            schema:
-              $ref: "#/components/schemas/EngineCommand"
-      responses:
-        "200":
-          description: Engine command result passed through.
-          content:
-            application/json:
-              schema:
-                $ref: "#/components/schemas/PassthroughObject"
-        "400":
-          $ref: "#/components/responses/InvalidRequestError"
-        "404":
-          $ref: "#/components/responses/NotFoundError"
-        "501":
-          $ref: "#/components/responses/NotImplementedError"
-        "500":
-          $ref: "#/components/responses/InternalError"
   /api/v1/user/games/{game_id}/orders:
     post:
       tags: [User]
@@ -3538,14 +3507,6 @@ components:
       properties:
         name:
           type: string
-    EngineCommand:
-      type: object
-      additionalProperties: true
-      description: |
-        Engine command request body. The schema is permissive because the
-        engine proxy passes the body through verbatim; the typed shape
-        lives in `pkg/model/rest.Command` and is enforced by
-        `internal/engineclient` before the engine call leaves backend.
     EngineOrder:
       type: object
       additionalProperties: true