Clean up Terraform: remove duplicate workspace-based config

- Remove root-level main.tf, variables.tf, outputs.tf, versions.tf
  (duplicated the module-based approach)
- Remove duplicate terraform/templates/ directory
- Remove stale terraform.tfvars.example and .terraform.lock.hcl
- Remove outdated scripts/import-production.sh
- Fix non-prod SMTP error by using merge() to completely omit
  SMTP fields instead of setting them to null
- Update README to remove workspace migration section

The environment directories approach (terraform/environments/*) is now
the only supported method.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

terraform/README.md

@@ -167,29 +167,3 @@ For team environments, configure a remote backend in each environment's `version
 - Use GitHub Secrets for CI/CD
 - OAuth provider credentials are set in Supabase Dashboard, not Terraform
 
-## Migration from Workspace-Based Setup
-
-If you previously used Terraform workspaces, migrate as follows:
-
-1. Export state from old setup:
-   ```bash
-   cd terraform  # old location
-   terraform workspace select production
-   terraform state pull > prod.tfstate
-   terraform workspace select non-production
-   terraform state pull > non-prod.tfstate
-   ```
-
-2. Import state to new environments:
-   ```bash
-   cd environments/prod
-   terraform init
-   terraform state push ../../prod.tfstate
-
-   cd ../non-prod
-   terraform init
-   terraform state push ../../non-prod.tfstate
-   ```
-
-3. Verify with `terraform plan` in each environment
-4. Delete old workspace-based files from terraform root