* Fix event loop blocking in NLTK downloads and Azure model listing
Found via watchdog detecting 61.6s hang during file upload.
**Root causes:**
1. NLTK punkt_tab downloads blocking during file processing
2. Azure model listing using sync requests.get() in async context
**Fixes:**
1. Pre-download NLTK data at Docker build time
2. Async fallback download at startup if build failed
3. Move Azure model fetch to thread pool with asyncio.to_thread()
**Impact:**
- Eliminates 60+ second event loop hangs
- Startup: instant if data baked in, ~60s async if needs download
- Requests: never block, all I/O offloaded to threads
* Fix Docker build: ensure /root/nltk_data exists even if download fails
- Create directory before download attempt
- Add verification step to confirm download success
- Directory always exists so COPY won't fail in runtime stage
* Fix: use venv python for NLTK download in Docker build
The builder stage installs NLTK in /app/.venv but we were using
system python which doesn't have NLTK. Now using venv python so
download actually works.
* Use uv run for NLTK download (more idiomatic)
uv run automatically uses the synced venv, cleaner than hardcoding
the venv path.
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