docs: add comprehensive Terraform state management guide

- State concepts, where secrets live, recovery matrix
- 4 collaboration options (TF Cloud, local, S3, manual) with comparison
- S3 backend full setup (bucket, DynamoDB lock, IAM policy, migration)
- Import commands for all Senzen resources with gotchas
- Disaster recovery procedures for each backend type
- DB password recovery (dashboard reset + terraform -replace)
- Daily workflow cheat sheets (TF Cloud, S3, local)
- Collaborator onboarding checklists for each option
- Full command reference and architecture diagram

Author: Long104