fix(presets): scope expand-prompt model resolution to owner (#3477)

* fix(presets): scope expand-prompt model resolution to owner

/api/presets/expand resolved its model endpoint with no owner, so in a
multi-user setup it could match another user's endpoint and use its URL
and decrypted api_key. Pass effective_user(request) to _resolve_model so
resolution is owner-scoped. Adds a regression test.

* fix(presets): scope teacher and audit model resolution to owner

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Alex Little <alexwilliamlittle@gmail.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Kenny Van de Maele <kenny@kvandemaele.be>

routes/preset_routes.py

@@ -9,6 +9,7 @@ from pydantic import BaseModel, Field
 
 from src.request_models import PresetUpdateRequest
 from core.middleware import require_admin
+from src.auth_helpers import effective_user
 
 logger = logging.getLogger(__name__)
 
@@ -100,7 +101,8 @@ def setup_preset_routes(preset_manager) -> APIRouter:
 
         try:
             model_spec = data.get("model") or ""
-            url, model, headers = _resolve_model(model_spec)
+            user = effective_user(request)
+            url, model, headers = _resolve_model(model_spec, owner=user)
             result = await llm_call_async(url, model, messages, temperature=0.8, max_tokens=500, headers=headers)
             return {"success": True, "prompt": result.strip()}
         except Exception as e: