DynamoDB Storage Backend

[raghavgopal]

What it adds:

• New module: idempotency-dynamodb
• Uses AWS SDK v2 DynamoDbClient
• Leverages DynamoDB conditional writes for atomic acquireOrGet
• Built-in TTL via DynamoDB's native TTL feature (no eviction scheduler needed)

Why it matters:

• AWS-heavy teams currently have no native option — they must use JDBC with RDS, which adds latency and operational overhead
• DynamoDB conditional expressions map perfectly to the idempotency acquire-or-get pattern