Adds an env config layer resolved from CDK context (`-c env=staging`, default prod)
and threads it through all three stacks so a fully isolated staging environment can
be deployed in the same AWS account.
prod is byte-identical: the prod config reproduces the deployed values exactly and
stackSuffix='' keeps every construct ID, stack name, and physical resource name
unchanged. Verified via synth — prod foundation keeps proposal-system-db /
-uploads / db-credentials / -auth / seahaven; staging suffixes all of them.
- config.ts: EnvConfig (prod + staging, same account) + resolveConfig
- app.ts: env-aware stack naming + config passthrough
- foundation/compute/frontend: ~40 physical names suffixed with config.stackSuffix;
CORS, Cognito domain/callbacks, alarms email from config; RETAIN / deletionProtection
gated on config.retainData so staging can be torn down
- cdk.json: register `env` context (default prod)
- post-deploy.sh: STACK_SUFFIX for dynamic stack-name lookup (default prod)
Note: automated staging CI deploy needs a one-line `cdk-context` input added to the
org reusable cd-cdk.yaml (companion change). prod deploy is unaffected (default prod).
