feat: Credits-System mit Verbrauchsanzeige im User-Dropdown

- DB-Migration: credits_total/credits_used/cost_per_credit auf licenses, token_usage_monthly Tabelle
- Orchestrator: Monatliche Token-Aggregation + Credits-Abzug nach Refresh
- Auth: Credits-Daten im /me Endpoint + Bugfix fehlende Klammer in get()
- Frontend: Credits-Balken im User-Dropdown mit Farbwechsel

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/models.py

@@ -41,6 +41,9 @@ class UserMeResponse(BaseModel):
     license_status: str = "unknown"
     license_type: str = ""
     read_only: bool = False
+    credits_total: Optional[int] = None
+    credits_remaining: Optional[int] = None
+    credits_percent_used: Optional[float] = None
 
 
 # Incidents (Lagen)