fix(infra): fix Lambda count errors and DynamoDB key_schema

[gpazevedo]

Summary

• Lambda: removed count = var.*_image_uri != "" ? 1 : 0 conditionals — Terraform cannot evaluate count when the value depends on ECR repo URLs unknown until apply. Lambdas are always created (image URI uses placeholder tag on first apply, replaced by CI/CD on deploy).
• Lambda outputs: removed corresponding [0] index references from outputs.
• DynamoDB: reverted key_schema block back to hash_key/range_key attributes — key_schema block syntax is not valid in AWS provider 6.36.

Test Plan

• terraform plan completes without errors
• terraform apply creates all three Lambda functions

🤖 Generated with Claude Code