feat(costs): $-Tagesbudget pro Nutzer (Stufe 3)

cost_daily_limit_usd (Pref > global; 0 = unbegrenzt) in quota.py
(cost_within_budget, cost_limit). ws.py-Gate: Web-Such- ODER Kostenbudget
aufgebraucht -> Web-Suche fuer den Turn aus + freundlicher Hinweis. Admin:
Schema-Feld + list_users + UI-Eingabe ($/Tag) + globales Live-Setting.

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

app/quota.py

@@ -68,3 +68,25 @@ def record_web_search(user, store, n: int = 1) -> None:
     """Zaehlt n tatsaechlich ausgefuehrte Web-Suchen fuer den Nutzer."""
     if n and n > 0 and user is not None:
         store.add_web_search_usage(user.id, n)
+
+
+def cost_limit(user, cfg=None) -> float:
+    """Max. Kosten/Tag (USD) fuer diesen Nutzer (Pref > global; 0 = unbegrenzt)."""
+    if cfg is None:
+        from app.runtime_config import runtime_settings
+        cfg = runtime_settings
+    pref = user.prefs.get("cost_daily_limit_usd") if user and user.prefs else None
+    if pref is not None:
+        try:
+            return float(pref)
+        except (TypeError, ValueError):
+            pass
+    return cfg.cost_daily_limit_usd
+
+
+def cost_within_budget(user, store, cfg=None) -> bool:
+    """Darf der Nutzer heute noch Kosten verursachen? (Limit 0 = unbegrenzt)."""
+    limit = cost_limit(user, cfg)
+    if not limit or limit <= 0:
+        return True
+    return store.cost_for_day(user.id) < limit