sec(iac): require AWS_IAM auth on production Lambda Function URL (closes #424)

[cristim]

Summary

• Set enable_cdn = true in github-prod.tfvars and github-staging.tfvars, which activates the full AWS_IAM auth chain for the Lambda Function URL in production and staging environments.
• The compute.tf local lambda_function_url_auth_type is derived from enable_cdn, so this single lever closes the exposure described in sec(iac): production Lambda Function URL uses authorization_type=NONE — no IAM gate #424 without touching any module logic.
• Dev environment retains enable_cdn = false (auth_type = NONE) because the browser-served SPA hits the Lambda URL directly and cannot SigV4-sign.

Auth chain (prod/staging with enable_cdn = true)

Browser -> CloudFront distribution
  -> OAC (aws_cloudfront_origin_access_control.lambda, signing_behavior = "always", sigv4)
    -> Lambda Function URL (authorization_type = "AWS_IAM")
      <- aws_lambda_permission.function_url_cloudfront
         (principal = cloudfront.amazonaws.com, source_arn = distribution ARN)

The Function URL rejects every request not SigV4-signed by the OAC-bearing distribution. Direct hits to the Function URL hostname return HTTP 403, even for requests with valid application-layer credentials.

The OAC resource, the CloudFront permission, and the derived auth-type local were all introduced in the base branch (feat/multicloud-web-frontend) as part of the preparatory refactor. This PR activates the mechanism for prod and staging by flipping enable_cdn = true.

Rollout notes

• Production and staging are not yet deployed (the lambda_allowed_origins still carry .invalid placeholder origins). Any accidental terraform apply will fail fast at hostname resolution until the operator sets real origins and custom domain names.
• When the environments are provisioned, set lambda_allowed_origins to the CloudFront distribution domain (or custom domain if configured) before applying.

Test plan

• Verify terraform fmt -check passes on changed files (CI)
• Verify tflint passes on terraform/environments/aws/ (CI)
• Confirm locals.lambda_function_url_auth_type = "AWS_IAM" for prod/staging plan output after enable_cdn = true
• Confirm aws_lambda_permission.function_url_cloudfront[0] is created (count = 1) in plan
• Confirm aws_lambda_permission.function_url[0] (principal = "*") is NOT created in plan (count = 0 when auth_type = AWS_IAM)
• Post-deploy: confirm direct HTTP request to Lambda Function URL returns 403
• Post-deploy: confirm browser via CloudFront returns 200

Closes #424

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Reviewing files that changed from the base of the PR and between 4956d66 and f36330b.

📒 Files selected for processing (2)

• terraform/environments/aws/github-prod.tfvars
• terraform/environments/aws/github-staging.tfvars

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• Create PR with unit tests
• Commit unit tests in branch fix/424-wave21

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