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/static/css/style.css

@@ -5335,3 +5335,61 @@ body.tutorial-active .tutorial-cursor {
     color: var(--text-primary);
     box-shadow: 0 0 0 2px rgba(150, 121, 26, 0.25);
 }
+
+/* ===== Credits-Anzeige im User-Dropdown ===== */
+.credits-section {
+    padding: 8px 16px 12px;
+}
+
+.credits-divider {
+    height: 1px;
+    background: var(--border);
+    margin-bottom: 10px;
+}
+
+.credits-label {
+    font-size: 11px;
+    font-weight: 600;
+    text-transform: uppercase;
+    letter-spacing: 0.5px;
+    color: var(--text-secondary);
+    margin-bottom: 6px;
+}
+
+.credits-bar-container {
+    width: 100%;
+    height: 6px;
+    background: var(--bg-tertiary);
+    border-radius: 3px;
+    overflow: hidden;
+    margin-bottom: 4px;
+}
+
+.credits-bar {
+    height: 100%;
+    border-radius: 3px;
+    background: var(--accent);
+    transition: width 0.6s ease, background-color 0.3s ease;
+    min-width: 2px;
+}
+
+.credits-bar.warning {
+    background: #e67e22;
+}
+
+.credits-bar.critical {
+    background: #e74c3c;
+}
+
+.credits-info {
+    font-size: 12px;
+    color: var(--text-secondary);
+    display: flex;
+    justify-content: center;
+    gap: 4px;
+}
+
+.credits-info span {
+    font-weight: 600;
+    color: var(--text-primary);
+}