feat: add model management endpoints and admin interface for cache status

Co-authored-by: Copilot <copilot@github.com>

frontend/app/main.py

@@ -390,6 +390,13 @@ def admin_delete_user(user_id: str):
     return redirect(url_for("admin"))
 
 
+@app.get("/admin/models")
+@admin_required
+def admin_models():
+    """Show model cache status and list all models."""
+    return render_template("admin/models.html")
+
+
 # ── Profile ───────────────────────────────────────────────────────────────
 
 @app.route("/users/profile", methods=["GET", "POST"])