Add Terraform for Augmentation Lambda

[nickclyde]

Summary

Closes #368

Adds Terraform resources to provision the augmentation Lambda and its supporting infrastructure, following the existing patterns for the TTC and index Lambdas.

• ECR repository (ttc-augmentation-lambda) for the container image built from Dockerfile.augmentation
• Lambda function with VPC config (only lambda_sg, no OpenSearch access needed), S3/region env vars, and lower memory/timeout defaults (512 MB, 300s) since no ML models are loaded
• Variables for augmentation-specific config (augmentation_lambda_function_name, augmentation_lambda_memory_size, augmentation_lambda_timeout, augmentation_lambda_image_tag)
• Outputs for ECR repository URL and Lambda function name
• README updated to document the augmentation Lambda resources

The augmentation Lambda reuses the shared lambda_role IAM role (consistent with the TTC and index Lambdas). SQS trigger infrastructure is out of scope here and tracked in #369.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.93%. Comparing base (1a8e8dd) to head (3a6979e).

Additional details and impacted files

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##             main     #403   +/-   ##
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  Coverage   93.93%   93.93%           
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  Files          42       42           
  Lines        2160     2160           
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  Hits         2029     2029           
  Misses        131      131           

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[robertandremitchell]

makes sense