docs: update DynamoDB on-demand pricing to current rates

[LeeroyHannigan]

Update DynamoDB on-demand pricing to current rates

AWS reduced DynamoDB on-demand throughput prices by 50%, effective November 1, 2024
(announcement). This file still lists the pre-cut rates, overstating on-demand
throughput cost by 2x.

	Before	After
Write request units	$1.25 /M	$0.625 /M
Read request units (strongly consistent)	$0.25 /M	$0.125 /M

Verifiable against the AWS Price List API, which is authoritative and versioned:

https://pricing.us-east-1.amazonaws.com/offers/v1.0/aws/AmazonDynamoDB/current/region_index.json

Scope of this change

Only the two on-demand throughput rates are changed. Deliberately not touched, because the
2024 price cut did not apply to them:

• Storage at $0.25/GB-month
• Provisioned capacity at $0.00065/WCU-hr and $0.00013/RCU-hr
• Transactional writes, which are $1.25/M -- the same figure as the stale standard-write rate
• Eventually-consistent reads, which are $0.0625/M post-cut

That last pair is the easy trap in review: $1.25/M is both the stale standard-write rate and the
correct transactional-write rate, and $0.125/M is both the stale eventual-read rate and the
correct strongly-consistent read rate. Each line changed here was read in context to confirm it
refers to standard on-demand throughput.

Knock-on totals

The stale rates live in the few-shot example inside cost_prompt, so they are fed to the model as a
reference exemplar rather than used in a direct calculation. The prompt above them already instructs
the model to consult the AWS price-changes page for current pricing, but a concrete worked example
is a much stronger anchor than that instruction, so the stale figures can surface in generated
estimates.

Two dependent figures in the example are updated to keep it self-consistent:

• DynamoDB row total: 25 GB x $0.25 = $6.25, plus 1M writes at $0.625/M = $0.625, plus 1M reads at
  $0.125/M = $0.125, giving $7.00 (was $7.75).
• Grand total: $169.08 (was $169.83), matching the new row sum exactly.

The $0.25 per GB-month storage rate in the same row is correct and is left unchanged.