Align S3 prefixes with APHL eCR pipeline

[nickclyde]

Summary

• Update 5 S3 prefix defaults to match APHL AIMS eCR pipeline conventions (ValidationResponseV2/, TextToCodeSubmissionV2/, TTCAugmentationMetadataV2/, TTCMetadataV2/, AugmentationMetadataV2/)
• Add source-bucket routing to TTC Lambda: extracts bucket name from S3 event with fallback to S3_BUCKET env var, enabling the training pipeline to share the same Lambda

Closes #374

Test plan

• All TTC Lambda tests pass (7/7)
• All Augmentation Lambda tests pass (7/7)
• Grep confirms no remaining references to old prefix strings
• Verify Terraform validates in CI
• Confirm prefix names with APHL

Follow-up

• Refactor augmentation Lambda to read from S3 instead of SQS message body #418: Refactor augmentation Lambda to read from S3 instead of SQS message body

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.93%. Comparing base (1a8e8dd) to head (19e5bdf).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #419   +/-   ##
=======================================
  Coverage   93.93%   93.93%           
=======================================
  Files          42       42           
  Lines        2160     2161    +1     
=======================================
+ Hits         2029     2030    +1     
  Misses        131      131           

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