Phase 4: Admin-Übersicht erweitern (Stats-Bar + Health-Badge inline + Letzter Treffer)

Backend
- routers/sources.py:
  - GET /api/sources/global/stats NEU: aggregierte Counter
    nach Typ, Total-Articles, Health-Bilanz (errors/warnings/ok)
  - GET /api/sources/global liefert pro Quelle health_status
    (worst-case error > warning > ok, NULL wenn nie gecheckt)

Frontend
- dashboard.html sub-global-sources: Stats-Bar Container oben.
  Tabellenkopf bekommt zwei neue Spalten: Letzter Treffer + Health.
- style.css: .sources-stats-bar (analog Monitor-Style),
  .health-badge mit Varianten error/warning/ok/unknown.
- sources.js:
  - loadGlobalSources lädt parallel /global + /global/stats
  - renderGlobalStats: rendert Stats-Bar mit Total-Quellen,
    Counts pro Typ (aus META), Total-Articles, Health-Counters
  - renderGlobalSources: 9 Spalten statt 7, Letzter-Treffer + Health-Badge,
    typeLabel statt TYPE_LABELS-Direktzugriff

src/static/dashboard.html

@@ -298,6 +298,7 @@
 
       <!-- Grundquellen -->
       <div class="section active" id="sub-global-sources">
+        <div class="sources-stats-bar" id="globalStatsBar"></div>
         <div class="action-bar">
           <div style="display:flex;align-items:center;gap:12px;flex-wrap:wrap;">
             <input type="text" class="search-input" id="globalSourceSearch" placeholder="Grundquelle suchen...">
@@ -327,6 +328,8 @@
                   <th class="sortable" data-sort="domain" onclick="sortGlobalSources('domain')">Domain <span class="sort-icon"></span></th>
                   <th class="sortable" data-sort="source_type" onclick="sortGlobalSources('source_type')">Typ <span class="sort-icon"></span></th>
                   <th class="sortable" data-sort="article_count" onclick="sortGlobalSources('article_count')">Artikel <span class="sort-icon"></span></th>
+                  <th class="sortable" data-sort="last_seen_at" onclick="sortGlobalSources('last_seen_at')">Letzter Treffer <span class="sort-icon"></span></th>
+                  <th class="sortable" data-sort="health_status" onclick="sortGlobalSources('health_status')">Health <span class="sort-icon"></span></th>
                   <th class="sortable" data-sort="status" onclick="sortGlobalSources('status')">Status <span class="sort-icon"></span></th>
                   <th>Aktionen</th>
                 </tr>