feat: initial phow2sim service

Tiny FastAPI service over PhoW2V Vietnamese word vectors. Mirrors
word2sim's endpoint shapes (/similarity /neighbors /vocab /random) so
clients can swap URLs without code changes.

- Auto-downloads VinAI's PhoW2V on first boot, caches binary .bin for ~5x faster restarts
- Viet-aware canonicalizer: exact -> lowercase -> space-to-underscore
- Supports both word (compound) and syllable variants via env
- Unicode-aware random-word filter accepts diacritics, rejects digits/punct

scripts/download-phow2v.sh

@@ -0,0 +1,52 @@
+#!/usr/bin/env bash
+# Download a PhoW2V variant from VinAI's public mirror into ./models/.
+# Usage: ./scripts/download-phow2v.sh [word|syllable] [100|300]
+# Defaults: word, 300.
+set -euo pipefail
+
+VARIANT="${1:-word}"
+DIMS="${2:-300}"
+
+case "$VARIANT" in
+  word)     SUFFIX="words" ;;
+  syllable) SUFFIX="syllables" ;;
+  *) echo "variant must be 'word' or 'syllable'" >&2; exit 2 ;;
+esac
+
+case "$DIMS" in
+  100|300) ;;
+  *) echo "dims must be 100 or 300" >&2; exit 2 ;;
+esac
+
+URL="https://public.vinai.io/word2vec_vi_${SUFFIX}_${DIMS}dims.zip"
+OUT_DIR="models"
+ZIP_PATH="${OUT_DIR}/word2vec_vi_${SUFFIX}_${DIMS}dims.zip"
+TXT_PATH="${OUT_DIR}/word2vec_vi_${SUFFIX}_${DIMS}dims.txt"
+
+mkdir -p "$OUT_DIR"
+
+if [[ -f "$TXT_PATH" ]]; then
+  echo "already present: $TXT_PATH"
+  exit 0
+fi
+
+echo "downloading $URL"
+curl -fL --progress-bar -o "$ZIP_PATH" "$URL"
+
+echo "extracting to $OUT_DIR"
+unzip -o -j "$ZIP_PATH" -d "$OUT_DIR"
+rm -f "$ZIP_PATH"
+
+# Unzip may produce a differently-named .txt depending on archive contents.
+# Rename to the expected path if a single .txt was extracted.
+if [[ ! -f "$TXT_PATH" ]]; then
+  shopt -s nullglob
+  candidates=("$OUT_DIR"/*.txt)
+  if [[ ${#candidates[@]} -eq 1 ]]; then
+    mv "${candidates[0]}" "$TXT_PATH"
+  else
+    echo "warning: could not resolve extracted .txt path; check $OUT_DIR" >&2
+  fi
+fi
+
+echo "ready: $TXT_PATH"