feat: fetch model via Nextcloud WebDAV with Basic auth

The upstream public.vinai.io mirror is dead and PhoW2V's research
license forbids public redistribution, so anonymous auto-download is
no longer viable. Expect a private Nextcloud (WebDAV or password-
protected public share) per deployment.

- Stream downloads in 1MiB chunks (flat RAM for ~1GB zips)
- Basic auth via MODEL_DOWNLOAD_USER / MODEL_DOWNLOAD_PASSWORD
- Drop the broken public.vinai.io default; compose requires MODEL_URL
- Add .env.example with WebDAV and public-share recipes
- Remove scripts/download-phow2v.sh (pointed at the dead mirror)
- README rewritten around the NC workflow; update license caveat

.env.example

@@ -0,0 +1,15 @@
+# Copy to `.env` and fill in your Nextcloud details. `.env` is gitignored.
+
+# --- WebDAV (recommended) ---
+MODEL_URL=https://cloud.example.com/remote.php/dav/files/yourname/phow2v/word2vec_vi_words_300dims.zip
+MODEL_DOWNLOAD_USER=yourname
+MODEL_DOWNLOAD_PASSWORD=xxxx-xxxx-xxxx-xxxx-xxxx   # Nextcloud app password
+
+# --- OR: password-protected public share ---
+# MODEL_URL=https://cloud.example.com/s/abc123XYZ/download
+# MODEL_DOWNLOAD_USER=
+# MODEL_DOWNLOAD_PASSWORD=your-share-password
+
+# --- Variant selection (must match the file you uploaded) ---
+MODEL_PATH=/data/phow2v/word2vec_vi_words_300dims.txt
+MODEL_VARIANT=word