Use Hetzner Cloud Placement Groups and labels to spread servers across physical hosts and target them in bulk

Documented steps

1. Create the group first: POST /v1/placement_groups with name and type. 'spread' is currently the only accepted type and guarantees the group's servers run on different physical hosts.
2. Attach at creation by passing placement_group: <id> to POST /v1/servers - this is the path to prefer.
3. To move an existing server, power it off, then POST /v1/servers/{id}/actions/add_to_placement_group with body {"placement_group": <id>}. Reverse it with remove_from_placement_group.
4. Label resources at create time or later with a labels object. Keys are an optional DNS-subdomain prefix plus a slash plus a required name segment of 63 chars or fewer, beginning and ending alphanumeric with dashes, underscores and dots between. Values follow the same 63-char rule and may be empty.
5. Filter list endpoints with label_selector, e.g. GET /v1/servers?label_selector=env%3D%3Dprod .
6. The selector grammar supports k==v and k=v, k!=v, bare k (key present), !k (key absent), k in (v1,v2), k notin (v1,v2), combined with commas: env==prod,!maintenance .
7. Reuse the same selectors as dynamic targets for firewall apply_to and load balancer targets so a correctly labelled new server automatically inherits both.
8. Reference: https://docs.hetzner.cloud/reference/cloud and https://docs.hetzner.com/cloud/placement-groups/overview/