Problem:
There is no frontend way to assign an arbitrary list of existing hosts
(e.g. a CSV from a CMDB, ticket or customer) to a host group. Today the
options are:

• Mass update, which only works on hosts selected via list filters and
    checkboxes, impractical when the hosts share no common attribute;
• Import, which requires full YAML/XML/JSON host definitions;
• API (hostgroup.massadd), which requires scripting and API access that
    many frontend users don't have.

Proposal:
Add "Add hosts from list" to the host group form and/or Mass update:
upload a CSV or paste a list of hosts, preview the result, apply.

Requirements (approximately):

1. Input: CSV upload or plain-text paste, one host per line.
2. Matching by selectable field: technical name, visible name, interface
      IP/DNS, or host ID.
3. Preview before applying: matched / not found / ambiguous (e.g. duplicate
      visible names), with only matched hosts applied.
4. Actions: add to group, remove from group, replace group membership.
5. RBAC: only hosts the user has write permission to are affected.
6. Every change recorded in the audit log.
7. Reasonable limits and paging for large lists (thousands of hosts).

Benefits:

• Faster onboarding of host sets from external sources without scripting
• Fewer errors than manual checkbox selection across filtered pages
• Makes group-based permissions, maintenance and actions easier to manage for users without API access