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/dashboard.html

@@ -50,6 +50,16 @@
                             <span class="header-dropdown-label">Lizenz</span>
                             <span class="header-dropdown-value" id="header-license-info">-</span>
                         </div>
+                        <div id="credits-section" class="credits-section" style="display: none;">
+                            <div class="credits-divider"></div>
+                            <div class="credits-label">Credits</div>
+                            <div class="credits-bar-container">
+                                <div id="credits-bar" class="credits-bar"></div>
+                            </div>
+                            <div class="credits-info">
+                                <span id="credits-remaining">0</span> von <span id="credits-total">0</span>
+                            </div>
+                        </div>
                     </div>
                 </div>
                 <div class="header-license-warning" id="header-license-warning"></div>