feat: add new space-diff-v2 table

[BravoNatalie]

According to this RFC#78 (related issue project-tracking#307)

This PR introduces a new space-diff table that uses cause as the sort key. The goal is to prevent duplicate diffs caused by past bugs (e.g. failed uploads), which led to inflated usage, slower queries, and “ghost” usage for spaces that should be empty after deletion.

Changes:

1. Create a new space-diff-v2 table with:
   • cause as the sort key
   • A GSI for timestamp-based queries
2. Enable dual writes:
   • On each diff event, write to both the existing table and the new table
   • Keep all readers (usage, reporting, billing) pointed at the existing table throughout January

For Future reference:
3. Cut over in February:

• Switch usage reporting and billing reads to the new table
• Keep the existing table as read-only historical storage

[seed-deploy]

View stack outputs

• pr592-warm-upload-api-BillingDbStack

  Name	Value
  customerTableName	pr592-warm-upload-api-customer
  egressTrafficTableName	pr592-warm-upload-api-egress-traffic-events
  spaceDiffArchiveTableName	pr592-warm-upload-api-space-diff-archive
  spaceDiffTableName	pr592-warm-upload-api-space-diff
  spaceDiffV2TableName	pr592-warm-upload-api-space-diff-v2
  spaceSnapshotTableName	pr592-warm-upload-api-space-snapshot
  usageTable	pr592-warm-upload-api-usage

• pr592-warm-upload-api-BillingStack

  Name	Value
  ApiEndpoint	https://ftdig32hm4.execute-api.us-east-2.amazonaws.com
  billingCronHandlerURL	https://x7z6jkdf2d2zhzpo2yv27eczpi0znypz.lambda-url.us-east-2.on.aws/
  CustomDomain	https://pr592.billing.warm.storacha.network
  EgressTrafficQueueURL	https://sqs.us-east-2.amazonaws.com/505595374361/pr592-warm-upload-api-egress-traffic-queue

• pr592-warm-upload-api-CarparkStack

  Name	Value
  BucketName	pr592-warm-upload-api-carpark-0
  Region	us-east-2

• pr592-warm-upload-api-RoundaboutStack

  Name	Value
  ApiEndpoint	https://staging.roundabout.web3.storage/
  CustomDomain	Using ROUNDABOUT_API_URL - no custom domain

• pr592-warm-upload-api-UcanInvocationStack

  Name	Value
  agentIndexBucketName	pr592-warm-upload-api-invocation-store-0
  agentMessageBucketName	pr592-warm-upload-api-workflow-store-0

• pr592-warm-upload-api-UploadApiStack

  Name	Value
  ApiEndpoints	["https://6ev8rmcayl.execute-api.us-east-2.amazonaws.com"]
  CustomDomains	["https://pr592.up.warm.storacha.network"]

• pr592-warm-upload-api-BusStack

• pr592-warm-upload-api-FilecoinStack

• pr592-warm-upload-api-UcanFirehoseStack

• pr592-warm-upload-api-UploadDbStack

[hannahhoward]

I'm not sure I understand why this can't be done with a GSI on cause.