feat(admin): Dropdown „Zentrales KI-Modell" — erprobte Presets ruck-zuck umstellen

Neue Karte in Admin › System › Status: ein Dropdown kuratierter, erprobter
Modelle, das Provider + Modell in einem Klick als Laufzeit-Override umstellt
(wirkt sofort, kein Neustart).

- Presets in config/models.yaml (Label, optional note, set:{key:value} aus
  RUNTIME_SETTABLE, default:true = Config-Standard). Loader app/model_presets.py
  (nur erlaubte LLM-Keys: default_llm_provider, openrouter_llm_model,
  local_llm_model).
- Endpunkte: GET /api/admin/models (Presets + aktuell aktives LLM + active_index),
  POST /api/admin/models/select (Default -> Overrides löschen, sonst Override-Keys
  setzen) — nutzt set_/delete_config_override + invalidate_cache, mit Audit-Log.
- Frontend: Karte zeigt Dropdown + „Aktiv: <provider> · <modell>"; Auswahl wirkt
  sofort und lädt den Status neu. Bump app.js v=52 -> v=53.
- Startliste: Mistral Small 24B (Standard), DeepSeek V4 Flash/Pro, Ollama Gemma 3.

Tests: Laden/Default-aktiv/Select-setzt-Override/Default-löscht/404/Auth.
263 Offline-Tests grün. Live via Headless-Chrome verifiziert (Auswahl ->
Override gesetzt, Provider-Karte spiegelt das Modell).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/api/admin.py

@@ -19,6 +19,7 @@ from app.audit import log_admin_action
 from app.auth import CAPABILITY_COOKIE, is_admin_user, require_admin, require_admin_or_user
 from app.config import settings
 from app.dependencies import get_store
+from app.model_presets import LLM_KEYS, load_presets
 from app.runtime_config import RUNTIME_SETTABLE, invalidate_cache, runtime_settings
 from app.schemas import AdminUserPrefsUpdate, MemoryCreate, MemoryOut, UserCreate, UserCreated, UserUpdate, UserAdminUpdate
 
@@ -451,6 +452,50 @@ async def delete_runtime_config(key: str, request: Request):
     return {"deleted": key}
 
 
+# ── Erprobte KI-Modelle (Dropdown) ─────────────────────────────────────────
+
+class ModelSelect(BaseModel):
+    index: int
+
+
+@router.get("/admin/models", dependencies=[Depends(require_admin)])
+async def list_model_presets():
+    """Kuratierte Modell-Presets + aktuell aktives LLM (für das Admin-Dropdown)."""
+    presets = load_presets()
+    effective = {k: str(getattr(runtime_settings, k, "") or "") for k in LLM_KEYS}
+    overrides = get_store().get_config_overrides()
+    no_llm_override = not any(k in overrides for k in LLM_KEYS)
+    active = None
+    for p in presets:
+        if p["is_default"]:
+            if no_llm_override:
+                active = p["index"]
+        elif p["set"] and all(effective.get(k) == v for k, v in p["set"].items()):
+            if active is None:
+                active = p["index"]
+    return {"presets": presets, "current": effective, "active_index": active}
+
+
+@router.post("/admin/models/select", dependencies=[Depends(require_admin)])
+async def select_model_preset(body: ModelSelect, request: Request):
+    """Wendet ein Preset an: Default -> LLM-Overrides löschen (Config-Standard);
+    sonst die Override-Keys des Presets setzen. Wirkt sofort (kein Neustart)."""
+    preset = next((p for p in load_presets() if p["index"] == body.index), None)
+    if preset is None:
+        raise HTTPException(status_code=404, detail=f"Preset {body.index} nicht gefunden.")
+    store = get_store()
+    if preset["is_default"]:
+        for key in LLM_KEYS:
+            store.delete_config_override(key)
+        log_admin_action(request, "model_preset_reset", label=preset["label"])
+    else:
+        for key, value in preset["set"].items():
+            store.set_config_override(key, value)
+        log_admin_action(request, "model_preset_set", label=preset["label"], set=preset["set"])
+    invalidate_cache()
+    return {"selected": preset["label"], "set": preset["set"], "is_default": preset["is_default"]}
+
+
 # ── Live-Log (journalctl → WebSocket) ──────────────────────────────────────
 
 @router.websocket("/admin/log")