Expose group membership (isMember) on the /v1/groups endpoints

[pinkustar]

first time ever contributing to this repo, so tell me if I am doing something wrong.
I studied the repo and got some help from Opus 4.8, I checked and verified all tests passed and the API endpoint worked locally

What

signal-cli's listGroups output includes an isMember flag indicating whether
the local account is still a member of a group. The REST wrapper read this into
SignalCliGroupEntry but dropped it when mapping to the public GroupEntry /
ExpandedGroupEntry types — only isBlocked was carried over.

As a result, GET /v1/groups/{number} and GET /v1/groups/{number}/{groupid}
returned every group signal-cli still knows about — including ones the account
has left or been removed from — with no way to distinguish them from groups
you're actively in.

Change

• Add a member boolean to GroupEntry and ExpandedGroupEntry, mirroring the
  existing blocked field, populated from signal-cli's isMember.
• Extract the per-entry mapping into signalCliGroupEntryToExpandedGroupEntry(...)
  and add a unit test covering isMember → member (and the independent
  isBlocked → blocked) mapping.
• Update the generated swagger (docs.go, swagger.json) for GroupEntry.member.

The field is additive — existing consumers are unaffected. It appears on both
endpoints, in both expand=false and expand=true responses.

GET /v1/groups/+1XXXXXXXXXX
[
  { "name": "Current Group", "blocked": false, "member": true,  "...": "..." },
  { "name": "Left Group",    "blocked": false, "member": false, "...": "..." }
]