fix(usage): preserve API key usage attribution in live stats

The 24h/today branch of getUsageStats keyed byApiKey buckets on the
masked key, while the daily rollup and the lastUsed overlay key on the
full key. Every key minted by one instance shares the sk-{machineId}
prefix, so the mask collapsed all of them into a single bucket and
attributed one key's usage to another.

Key the live branch by the full api key too; the masked value is still
carried on the bucket for display.