fix: isolate Web Push device failures (Closes #655)

src/push_notifications.py

@@ -219,8 +219,17 @@ async def dispatch_unread_updates(
                     count += 1
                 return count
 
+        async def dispatch_device_safely(delivery) -> int:
+            try:
+                return await dispatch_device(delivery)
+            except Exception:
+                # Device-specific validation, persistence, or provider failures
+                # must not cancel healthy siblings. Leave any uncheckpointed
+                # revisions unseen so a later poll can retry them.
+                return 0
+
         counts = await asyncio.gather(
-            *(dispatch_device(delivery) for delivery in deliveries)
+            *(dispatch_device_safely(delivery) for delivery in deliveries)
         )
         return sum(counts)
     finally: