Fix audit export totals JSON keys

[Sebastefanelli]

Summary

• Add JSON tags to storage.LedgerTotals
• Keep riskkernel audit export totals consistent with the rest of the JSON output
• Add a regression test that rejects capitalized Go field names in the serialized totals object

Fixes #30

Testing

• go test ./internal/storage -run TestLedgerTotalsJSONTags -count=1
• go test ./...

[prashar32]

Thanks so much for this, @Sebastefanelli - and sorry for the duplicate effort! #35 landed the same fix a bit earlier, so I'm going to honor first-in and merge that one. Your standalone marshal test was clean, though - appreciated the care. If you'd like to keep contributing I'd genuinely welcome it: #38 (surfacing the tool_calls audit trail via an API/CLI) is a nicely-scoped next one, and there are more under the good first issue label. Thanks again for jumping in 🙏