Add a reliable MongoDB replica setup runbook

[tdurieux]

Summary

• replace the abbreviated instructions with a complete production runbook covering backup, Tailscale, both environment files, key distribution, conversion, initial sync, application cutover, operations, and rollback
• add deterministic Docker aliases for the advertised mongo-primary and mongo-secondary member names
• map cross-server names explicitly to Tailscale addresses instead of relying on container MagicDNS behavior
• add preflight endpoint checks and wait for a writable primary before allowing the next step
• document fixes for the keyfile, missing --replSet, hostname self-mapping, connectivity, and application URI errors

Validation

• shell syntax and ShellCheck (when installed)
• primary and secondary Docker Compose rendering
• required hostname/address interpolation
• whitespace checks

Follow-up to #765 after validating the procedure against the setup failures encountered during deployment.