mongodb: Add indexes for events

Events are awfully slow even on ordinary request like:
https://staging.kernelci.org:9000/latest/events?kind=job&limit=1000&from=2025-12-11T13:00:00

This index should help with that.

Signed-off-by: Denys Fedoryshchenko <denys.f@collabora.com>

Author: nuclearcat

api/pubsub_mongo.py

@@ -201,6 +201,11 @@ async def _ensure_indexes(self):
             [('channel', ASCENDING), ('sequence_id', ASCENDING)],
             name='channel_sequence_id'
         )
+        # Compound index for filtered event queries (kind + timestamp)
+        await event_col.create_index(
+            [('data.kind', ASCENDING), ('timestamp', ASCENDING)],
+            name='kind_timestamp'
+        )
 
         # Subscriber state indexes
         # Unique index on subscriber_id