Add PostgreSQL and MS-SQL database backup guides for the tenancy/vm module

Description

The tenancy/vm examples now document how to back up a VM's plain files with restic (#231 / #232), but not how to back up a database running on that VM. Logical/native database backups are portable across hosts and tolerant of same-or-newer restore targets, which a raw data-directory copy (restic) is not — so a database needs its own guide using the engine's native tooling.

This is the database counterpart to the existing VM-data (restic) and cluster (etcd + Velero) backup guides.

Proposal

Add two self-service guides beside the restic one, matching the style of k8s-cluster/examples/BACKUPS.md and vm/examples/BACKUPS.md:

• modules/tenancy/vm/examples/POSTGRES-BACKUPS.md — logical dumps (pg_dumpall --roles-only + per-database pg_dump -Fc) streamed to the tenant's own S3 bucket under timestamped prefixes; in-guest systemd timer via the vm module's user_data (base64-embedded scripts); restore into a fresh VM (same DC), a single database, and cross-DC DR.
• modules/tenancy/vm/examples/MSSQL-BACKUPS.md — native BACKUP DATABASE ... TO DISK + aws s3 cp, with server logins exported separately (SID + password hash) and contained-database auth enabled on restore; same install/verify/restore shape.

Each includes a scheduling/retention reference and a production-hardening checklist, and cross-links the restic and cluster guides.

Start with k8s-cluster/examples/BACKUPS.md and vm/examples/BACKUPS.md, then inspect the tenancy/vm examples and user_data patterns. Add POSTGRES-BACKUPS.md and MSSQL-BACKUPS.md covering installation, verification, scheduling and retention, production hardening, restore scenarios, and cross-links to the existing guides.