refactor(deploy): make DICT_VERSION a required build arg (single-sourced)

Drop the literal version default from the build files (backend Dockerfile both
stages, loadtest Dockerfile, the compose build-arg) so the release tag is not
duplicated as a stale-prone default a newcomer can't tell from the real source.

DICT_VERSION is now required: compose uses ${DICT_VERSION:?…} and the Dockerfiles
have no ARG default, so a missing value fails loudly instead of baking a stale tag.
The tag lives only in its genuine sources — ci.yaml env (CI tests), the Gitea
TEST_/PROD_DICT_VERSION variables (deploy seed) and deploy/.env.example (local).
Adds a "Bumping the dictionary version" section to deploy/README and fixes the bare
docker-build examples (CLAUDE.md, README.md, loadtest/README) to pass --build-arg.

loadtest/README.md

@@ -35,8 +35,8 @@ The harness reaches Postgres and the gateway directly, so run it as a one-shot
 container on the contour's docker network (this bypasses the host→gateway hairpin):
 
 ```sh
-# from the repo root
-docker build -f loadtest/Dockerfile -t scrabble-loadtest .
+# from the repo root (DICT_VERSION has no default — pass the scrabble-dictionary release tag)
+docker build --build-arg DICT_VERSION=v1.3.0 -f loadtest/Dockerfile -t scrabble-loadtest .
 
 docker run --rm --cpus=3 --name scrabble-loadtest --network scrabble-internal \
   -e POSTGRES_PASSWORD="$TEST_POSTGRES_PASSWORD" \