fix: handleKeyDown type cast, Terraform locking, sensitive outputs, password validation

[Mozez155]

Summary

• handleKeyDown event cast is incorrect in LandingPage component #952 — Replace incorrect as unknown as React.FormEvent cast in LandingPage.handleKeyDown with a properly typed React.KeyboardEvent<HTMLFormElement> handler that calls requestSubmit(); add unit tests covering Enter submission, validation on empty input, and non-Enter key no-op.
• Terraform remote state DynamoDB lock table not configured — concurrent applies risk corruption #953 — Pass environment-specific backend HCL files to terraform init so DynamoDB state locking is actually activated; add -lock=true -lock-timeout=5m to all terraform plan and terraform apply CI steps; add a terraform validate step before every plan.
• Terraform outputs expose sensitive values without sensitive = true #954 — Add sensitive = true to the Redis endpoint and redis_url module outputs and to redis_endpoint in the root outputs.tf, preventing plaintext leakage in CI logs and terraform output.
• No input validation on Terraform sensitive variables — weak passwords accepted #955 — Raise db_password minimum length from 8 → 24 characters and add complexity rules (upper, lower, digit, special); add redis_auth_token variable with sensitive = true and the same validation; add a CI terraform validate step backed by a placeholder var file.

Test plan

• TypeScript compiles cleanly in frontend/ with no type errors
• LandingPage.keyboard.test.tsx passes (Enter submits, empty-email shows error, non-Enter is a no-op)
• terraform validate -var-file=environments/ci-validate.tfvars exits 0
• terraform plan in CI now passes -lock=true -lock-timeout=5m and uses the correct backend HCL per environment
• terraform output masks redis_endpoint as <sensitive>
• Supplying a db_password or redis_auth_token shorter than 24 chars causes terraform validate to fail with the new error message

closes #952
closes #953
closes #954
closes #955

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