Release 0.16.0: owner-only invite for per-RFC contribution + discussion

Lands roadmap item #12: the RFC's owner can invite specific users by
email to one of two per-RFC roles — contributor (open PRs and join
discussion) or discussant (join discussion only). Non-invited users
keep the v0.6.0 anonymous-read contract: they can read but cannot
write/discuss that RFC. The platform-level grant (v0.8.0 / item #6)
is unchanged; this release adds a per-RFC membership layer beneath
it.

Migration 018_rfc_invitations.sql adds rfc_invitations (the
lifecycle row with the email token and 30-day expiry) and
rfc_collaborators (the accepted-invitation substrate the write
gate consults). FK-cascaded against cached_rfcs and users per §5.

New endpoints (in backend/app/api_invitations.py):
  POST   /api/rfcs/{slug}/invitations              (owner)
  GET    /api/rfcs/{slug}/invitations              (owner)
  POST   /api/rfcs/{slug}/invitations/{id}/revoke  (owner)
  GET    /api/invitations/accept?token=…           (signed-in)
  POST   /api/invitations/accept                   (signed-in)

The discussion / branch / open-pr write surfaces compose new
auth.can_discuss_rfc and auth.can_contribute_to_rfc predicates
after the existing require_contributor check. A super-draft with
no frontmatter owners yet falls through to the platform-granted
contract — the gate engages only once an owner exists.

The email path reuses the existing SMTP plumbing (EmailConfig.from_env)
the v0.7.0 OTC and v0.5.0 notification mailers share — transactional
envelope, no preferences honored, no unsubscribe footer.

The §17 admin user-management surface (item #7, v0.9.0) is extended
additively: GET /api/admin/users carries a new per-user
rfc_invitations array naming each accepted per-RFC collaboration
with inviter / RFC / role / timestamp. The platform-grant decision
keeps its context without restructuring the existing shape.

Frontend additions: InvitationsModal.jsx (owner-only RFC view
header strip affordance), AcceptInvitation.jsx (the
/invitations/accept landing page), five api.js helpers.

237 backend tests pass (18 new in test_rfc_invitations_vertical.py,
three older tests updated to opt into the per-RFC contributor
contract via the new grant_rfc_collaborator test seam). Frontend
build clean.

No new env vars. No new overlay keys. Migration auto-applied on
backend start by the existing db.run_migrations() sweep.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/tests/test_graduation_vertical.py

@@ -34,6 +34,7 @@ import pytest
 from test_propose_vertical import (  # noqa: F401
     FakeGitea,
     app_with_fake_gitea,
+    grant_rfc_collaborator,
     provision_user_row,
     sign_in_as,
     tmp_env,
@@ -248,6 +249,9 @@ def test_graduate_refuses_when_body_edit_pr_open(app_with_fake_gitea):
         provision_user_row(user_id=2, login="alice", role="contributor")
         seed_owned_super_draft(fake, slug="ohm", title="OHM",
                                pitch=PITCH, owners=["ben"])
+        # v0.16.0 (item #12): ben is the RFC owner; alice needs a per-RFC
+        # contributor invitation to cut an edit branch on the super-draft.
+        grant_rfc_collaborator(user_id=2, rfc_slug="ohm", role_in_rfc="contributor")
         sign_in_as(client, user_id=2, gitea_login="alice",
                    display_name="Alice", role="contributor")
 
@@ -497,6 +501,8 @@ def test_pre_graduation_history_surfaces_edit_branch_threads(app_with_fake_gitea
         provision_user_row(user_id=2, login="alice", role="contributor")
         seed_owned_super_draft(fake, slug="ohm", title="OHM",
                                pitch=PITCH, owners=["ben"])
+        # v0.16.0 (item #12): alice needs per-RFC contributor access.
+        grant_rfc_collaborator(user_id=2, rfc_slug="ohm", role_in_rfc="contributor")
 
         # Alice cuts an edit branch and starts chatting on it.
         sign_in_as(client, user_id=2, gitea_login="alice",