fix(ci): prod build needs EXPORT_SIGN_KEY for compose interpolation

The first v1.8.0 prod-deploy build job failed: `docker compose build` interpolates the
WHOLE compose file, and the backend's :?-guarded EXPORT_SIGN_KEY — added with the
finished-game export after the last prod release (v1.7.0), so the prod build never
exercised it — was absent from the build job's env, tripping the guard before any image
built. Prod was untouched (build-only failure: no image pushed, no deploy, no migration).

Add EXPORT_SIGN_KEY (audited every :?-guarded compose var against the build job env — it
was the only genuinely-missing one; TELEGRAM_MINIAPP_URL is derived in the run step).
Verified by reproducing the build-job env + `docker compose config`.

.gitea/workflows/prod-deploy.yaml

@@ -46,6 +46,12 @@ jobs:
       # VITE_GATEWAY_URL omitted: the SPA is served same-origin (compose ":-" default).
       POSTGRES_PASSWORD: ${{ secrets.PROD_POSTGRES_PASSWORD }}
       GM_BASICAUTH_HASH: ${{ secrets.PROD_GM_BASICAUTH_HASH }}
+      # `docker compose build` interpolates the WHOLE compose file, so every :?-guarded
+      # runtime var must be present at build even though it is not a build-arg — incl. the
+      # backend's EXPORT_SIGN_KEY (added with the finished-game export after v1.7.0, which is
+      # why the first v1.8.0 build tripped on it). POSTGRES_PASSWORD/GM_BASICAUTH_HASH above
+      # are here for the same reason; DICT_VERSION + the derived Mini App URL cover the rest.
+      EXPORT_SIGN_KEY: ${{ secrets.PROD_EXPORT_SIGN_KEY }}
       # PUBLIC_BASE_URL drives the derived VK ID redirect (baked into the SPA) and the
       # Mini App URL; both are computed in the build step, not stored variables.
       PUBLIC_BASE_URL: ${{ vars.PROD_PUBLIC_BASE_URL }}