Feat/channel invitation workflow part 3

[Pranav-d33]

Adds invitation endpoints to the Channel API — replaces the one-step add_organization flow with a proper Fabric channel update workflow where current members create artifacts, collect signatures, and invitees join.

What's Implemented

New Endpoints (on /api/v1/channels/{channel_id})

Method	URL	Action	Auth
GET	/invitations	List invitations visible to org	Any member
POST	/invitations	Create invitation + generate artifact	Admin
GET	/invitations/{id}	Retrieve invitation details	Visibility-gated
POST	/invitations/{id}/cancel	Cancel invitation	Admin or invited org

Design Decisions

Question	Decision
Admin-only mutations?	Yes — create/cancel require is_admin. List/retrieve is any member (visibility-gated).
Signature threshold	All current members by default. Creator can override via required_signatures.
One or multiple invitees?	Multiple — organization_ids accepts a list.
Retry on failure?	No — cancel and re-create.
Cancelable states?	DRAFT, SIGNING, FAILED, READY — not once ACCEPTED/REJECTED/CANCELED.
Cancel vs Reject (PR 6)?	Cancel hard-stops the whole invitation. Reject is per-invitee decline.
Who can cancel?	Channel member admin (any org) or invited org (only their own invitation).

Key Logic

• Artifact generation: create_invitation_artifact() calls the creator org's agent at POST /channels/{name}/invitations/definition to produce the unsigned channel update artifact. Agent endpoint TBD (PR 4) — service function is written and tested with mocks.
• No partial state: Agent is called first (outside transaction). DB records created only on success.
• Duplicate protection: Rejects creation if any invitee org already has a PENDING invitee record on an active (DRAFT/SIGNING/READY) invitation.
• Visibility: Members see all invitations on their channels. Invitees see only READY+/ACCEPTED/REJECTED/FAILED. Unrelated orgs see nothing.

Files Changed

File	Change
src/api-engine/channel/views.py	3 @action methods on ChannelViewSet
src/api-engine/channel/serializers.py	Updated ChannelInvitationCreateBody.create(), new ChannelInvitationCancelSerializer
src/api-engine/channel/service.py	New create_invitation_artifact() agent call
src/api-engine/channel/tests.py	12 endpoint tests (APIClient + JWT)

How to Test

cd src/api-engine
python manage.py test channel
Stacking
This PR is stacked on feat/channel-invitation-workflow-part-2 (models + serializers). The PR diff against main shows only the new endpoint changes. Next in stack: PR 4 (Fabric agent artifact generation endpoints).