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/scraper/wikidata_breeds.py

@@ -165,10 +165,10 @@ async def mirror_wikidata_photos():
 
             # Wikimedia Commons: append ?width=600 for scaled download
             fetch_url = img_url if "?" in img_url else img_url + "?width=600"
-            retries = 2
+            retries = 4
             for attempt in range(retries):
                 try:
-                    await asyncio.sleep(0.3)   # 300ms zwischen Requests → ~3/s
+                    await asyncio.sleep(1.0 * (attempt + 1))  # exponentiell: 1s, 2s, 3s, 4s
                     r = await client.get(fetch_url)
                     if r.status_code == 200 and r.headers.get("content-type", "").startswith("image"):
                         with open(local_path, "wb") as f: