fix: seed admin user on startup and run sync as background task

- Seed/update admin user from ADMIN_USERNAME/ADMIN_PASSWORD settings on
  startup (admin_users table was empty, login was impossible)
- POST /api/admin/sync now returns immediately and runs the equipment
  sync via BackgroundTasks (full sync took minutes and hit nginx 504)
- run_sync accepts optional sync_log_id to reuse a pre-created log entry
- Adapt admin router test to background sync behavior

backend/app/services/sync_service.py

@@ -23,10 +23,10 @@ class SyncService:
         self.db = db
         self.rentman = rentman or RentmanService()
 
-    async def run_sync(self) -> dict[str, Any]:
+    async def run_sync(self, sync_log_id: int | None = None) -> dict[str, Any]:
         """Execute an incremental equipment sync.
 
-        1. Create sync_log entry (status=running)
+        1. Create sync_log entry (status=running) or reuse sync_log_id
         2. Paginate GET /equipment from Rentman
         3. Compare updateHash with DB, only process changed items
         4. Download images only for changed items
@@ -35,12 +35,20 @@ class SyncService:
         7. Update sync_log (status=completed or failed)
         Returns dict with sync_id, items_processed, status.
         """
-        log_entry = SyncLog(
-            sync_type="equipment",
-            status="running",
-            started_at=datetime.utcnow(),
-        )
-        self.db.add(log_entry)
+        if sync_log_id is not None:
+            log_entry = await self.db.get(SyncLog, sync_log_id)
+            if not log_entry:
+                log_entry = None
+        if sync_log_id is None or log_entry is None:
+            log_entry = SyncLog(
+                sync_type="equipment",
+                status="running",
+                started_at=datetime.utcnow(),
+            )
+            self.db.add(log_entry)
+        else:
+            log_entry.status = "running"
+            log_entry.started_at = datetime.utcnow()
         await self.db.commit()
         await self.db.refresh(log_entry)
         sync_id = log_entry.id