perf(images): offload validate_url() DNS resolution with asyncio.to_thread (#25825)

validate_url() calls socket.getaddrinfo() for SSRF protection, which
blocks the event loop for 100-700ms per DNS lookup. This affects:

- Image generation (get_image_data) — every external image URL
- Image editing (load_url_image) — every external image URL
- OAuth profile pictures (_process_picture_url) — every login
- Webhook delivery (post_webhook) — every notification
- Image base64 conversion (get_image_base64_from_url) — chat images

Wrap all 5 async call sites in asyncio.to_thread() so DNS resolution
runs in the thread pool. The event loop remains free to serve other
requests during the lookup.

Benchmark (3 domains, 3 trials averaged):
- BEFORE: max jitter 479ms, 1 blocked ping per trial
- AFTER:  max jitter 1ms, 0 blocked pings (324x improvement)

Co-authored-by: Tim Baek <tim@openwebui.com>

backend/open_webui/utils/oauth.py

@@ -1,3 +1,4 @@
+import asyncio
 import base64
 import fnmatch
 import hashlib
@@ -1603,7 +1604,7 @@ class OAuthManager:
             return '/user.png'
 
         try:
-            validate_url(picture_url)
+            await asyncio.to_thread(validate_url, picture_url)
 
             get_kwargs = {}
             if access_token: