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/Dockerfile

@@ -12,7 +12,8 @@
 
 # --- dictionary artifact -----------------------------------------------------
 FROM alpine:3.20 AS dawg
-ARG DICT_VERSION=v1.3.0
+# Required, no default: the build caller supplies the scrabble-dictionary release tag.
+ARG DICT_VERSION
 RUN apk add --no-cache curl tar
 RUN mkdir -p /dawg \
  && curl -fsSL -o /tmp/dawg.tar.gz \