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.

deploy/docker-compose.yml

@@ -68,9 +68,11 @@ services:
       context: ..
       dockerfile: backend/Dockerfile
       args:
-        # Seed dictionary for a FRESH volume; the per-contour value comes from the
-        # deploy env (Gitea TEST_/PROD_DICT_VERSION). See the volume note below.
-        DICT_VERSION: ${DICT_VERSION:-v1.3.0}
+        # Seed dictionary for a FRESH volume; required (no default) so the release tag is
+        # set in exactly one place per context — the deploy env (Gitea TEST_/PROD_DICT_VERSION)
+        # or .env for local builds. See the volume note below + deploy/README.md "Bumping the
+        # dictionary version".
+        DICT_VERSION: ${DICT_VERSION:?set DICT_VERSION — the scrabble-dictionary release tag, e.g. in deploy/.env}
         # Build version stamped into the binary (git tag; see pkg/version).
         VERSION: ${APP_VERSION:-dev}
     restart: unless-stopped