Stage 16: deploy/README.md — full environment-variable reference

- deploy/README.md documents the services, how to run it locally and in CI, and
  every variable: required (the four :? ones + ≥1 bot token) and optional with
  defaults, marked secret-vs-variable and with the TEST_/PROD_ Gitea mapping;
  plus the fixed internal wiring and the host-side setup.
- ci.yaml maps the remaining POSTGRES_DB/USER, DICT_VERSION and LOG_LEVEL (unset
  renders empty -> the compose ":-" defaults apply), so every documented var is
  per-contour overridable.
- .env.example points at the README for the full reference.

deploy/.env.example

@@ -1,6 +1,8 @@
 # Environment for deploy/docker-compose.yml. The CI deploy job (ci.yaml) maps the
 # Gitea TEST_-prefixed secrets/variables onto these unprefixed names; Stage 17
 # maps the PROD_-prefixed set the same way. Copy to deploy/.env for a local run.
+#
+# Full reference (required vs optional, defaults, secret-vs-variable): deploy/README.md.
 
 # --- Postgres ---------------------------------------------------------------
 POSTGRES_DB=scrabble