fix: Remove nltk blocking download and make azure model listing non-blocking (#6214)

* 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.

letta/server/rest_api/app.py

@@ -163,6 +163,18 @@ async def lifespan(app_: FastAPI):
     except Exception as e:
         logger.warning(f"[Worker {worker_id}] Failed to start watchdog: {e}")
 
+    # Pre-download NLTK data to avoid blocking during requests (fallback if Docker build failed)
+    try:
+        import asyncio
+
+        import nltk
+
+        logger.info(f"[Worker {worker_id}] Checking NLTK data availability...")
+        await asyncio.to_thread(nltk.download, "punkt_tab", quiet=True)
+        logger.info(f"[Worker {worker_id}] NLTK data ready")
+    except Exception as e:
+        logger.warning(f"[Worker {worker_id}] Failed to download NLTK data: {e}")
+
     if telemetry_settings.profiler:
         try:
             import googlecloudprofiler