Sprint 16: Chat-Fotos/Online/Read-Receipts, Gesundheit-Dokumente löschen, Bugfixes

- Chat: Foto-Versand (POST /api/chat/conversations/{id}/upload, media_url/media_type)
- Chat: Online-Indikator (last_seen Heartbeat, grüner Dot, 3min-Fenster)
- Chat: Read Receipts (read_at, Einzel-/Doppelhaken-Icons)
- Gesundheit: Dokument löschen (DELETE .../dokument, Datei + DB-Eintrag)
- Bug: events.user_id NOT NULL → nullable (Table-Recreation-Migration)
- Bug: scheduler INSERT user_id 0 → NULL
- Bug: Wikidata Rate-Limit: sleep 0.3s→1.0s, retries 2→4, exponentielles Backoff
- SW: by-v146, APP_VER 119

backend/static/js/api.js

@@ -126,6 +126,9 @@ const API = (() => {
     uploadDokument(dogId, id, formData) {
       return upload(`/dogs/${dogId}/health/${id}/dokument`, formData);
     },
+    deleteDocument(dogId, id) {
+      return del(`/dogs/${dogId}/health/${id}/dokument`);
+    },
     kiZusammenfassung(dogId) {
       return post(`/dogs/${dogId}/health/ki-zusammenfassung`);
     },
@@ -344,6 +347,12 @@ const API = (() => {
     send(convId, text)          { return post(`/chat/conversations/${convId}/messages`, { text }); },
     markRead(convId)            { return post(`/chat/conversations/${convId}/read`, {}); },
     deleteMessage(msgId)        { return del(`/chat/messages/${msgId}`); },
+    uploadPhoto(convId, file)   {
+      const fd = new FormData();
+      fd.append('file', file);
+      return upload(`/chat/conversations/${convId}/upload`, fd);
+    },
+    heartbeat()                 { return post('/chat/heartbeat', {}); },
   };
 
   // ----------------------------------------------------------