feat: add IFRAME_CSP env var for srcdoc iframe content security policy

Adds an IFRAME_CSP environment variable that injects a Content-Security-Policy
<meta> tag into all srcdoc iframes rendering untrusted content:
- Artifacts (LLM-generated HTML previews)
- FullHeightIframe (tool/embed output)
- FilePreview (user-uploaded HTML files)
- CitationModal (RAG document HTML)

Shared utility in src/lib/utils/csp.ts handles injection with HTML-safe
attribute escaping. URL-based iframes (src=) are correctly excluded.

Env-var only — no PersistentConfig, no admin UI, no DB. Set once at deploy
time, requires restart. Empty string (default) means no CSP restriction.

backend/open_webui/main.py

@@ -460,6 +460,7 @@ from open_webui.config import (
     OAUTH_PROVIDERS,
     WEBUI_URL,
     RESPONSE_WATERMARK,
+    IFRAME_CSP,
     # Admin
     ENABLE_ADMIN_CHAT_ACCESS,
     ENABLE_ADMIN_ANALYTICS,
@@ -2444,6 +2445,7 @@ async def get_app_config(request: Request):
                     'pending_user_overlay_title': app.state.config.PENDING_USER_OVERLAY_TITLE,
                     'pending_user_overlay_content': app.state.config.PENDING_USER_OVERLAY_CONTENT,
                     'response_watermark': app.state.config.RESPONSE_WATERMARK,
+                    'iframe_csp': IFRAME_CSP,
                 },
                 'license_metadata': app.state.LICENSE_METADATA,
                 **(