Add missing indexes on wallet_transactions

Context

In two recent MR reviews, a couple of indexes were identified as being useful for the wallet_transactions DB table.

• created_at: This is used for filtering and sorting (example)
• consumer_id: This is used for filtering (example)

Follow-ups from MRs

The following discussion from !14886 should be addressed:

• @tyleramos started a discussion:

  Suggestion (non-blocking): I'm not seeing an index on the transactions table for the created_at column. We might want to consider adding one since we are ordering on it here. We could probably do this as a follow-up if you prefer. Also I see a similar query existed previously without the index so again not a blocker from me.

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The following discussion from !14354 should be addressed:

• @vij started a discussion:

  non-blocking: we might need to consider adding an index for this? non-blocking as we can add it as a follow-up if so after some monitoring or if it becomes identified as problematic/slow