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.

README.md

@@ -89,7 +89,7 @@ observability stack (OTel Collector → Prometheus + Tempo → Grafana) + a fron
 services build from multi-stage distroless `*/Dockerfile`.
 
 ```sh
-docker build -f backend/Dockerfile -t scrabble-backend .   # pulls the DAWG release artifact
+docker build --build-arg DICT_VERSION=v1.3.0 -f backend/Dockerfile -t scrabble-backend .  # DICT_VERSION required; pulls that DAWG release artifact
 docker build -f gateway/Dockerfile -t scrabble-gateway .   # node stage builds + embeds the UI
 docker compose -f deploy/docker-compose.yml config         # validate (needs the TEST_/PROD_ env)
 ```