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/files.py

@@ -58,7 +58,7 @@ async def get_image_base64_from_url(url: str, user=None) -> Optional[str]:
             # called only on the originally-submitted URL; following 3xx redirects
             # without re-validation would let an attacker reach private IPs via a
             # public host that redirects internally (e.g. cloud-metadata exfil).
-            validate_url(url)
+            await asyncio.to_thread(validate_url, url)
             # Fetch through an SSRF-safe session that re-checks the connect-time IP, so a
             # rebinding DNS answer that passed validate_url cannot reach an internal address.
             async with get_ssrf_safe_session() as session: