refactor: remove unused MODEL_VARIANT env var

Was set in Dockerfile/compose/.env/README but never read by the app.
Tokenization is inferred at lookup time by _variant_candidates trying
both spaced and underscore-joined forms.

.env.example

@@ -10,6 +10,5 @@
 #   MODEL_URL=https://cloud.example.com/s/abc123XYZ/download
 MODEL_URL=https://cloud.example.com/s/REPLACE_ME/download
 
-# Variant — must match the filename inside the zip.
+# Must match the filename inside the zip.
 MODEL_PATH=/data/phow2v/word2vec_vi_words_300dims.txt
-MODEL_VARIANT=word