revert(hub): M6 — add_group_member must keep accepting node tokens

M6 in the third review was a misread. `add_group_member` accepting a
node-scoped token is deliberate (commit 0443cf8): the node calls
POST /v1/groups/{id}/members/{username} after a CLI `member invite` so
the group shows up in the invitee's SPA, authenticating with a
node-scoped token. `group.admin_id == caller` is the real guard.

An older test (`test_node_scope_blocks_add_member`) asserted the
opposite and had been left red on main; the M6 "fix" (commit 6b38704)
satisfied that test by switching the dependency to `require_user_scope`
— which made `ops.create_invite`'s hub-membership call 403. That
exception is swallowed with a log.warning, so an invited user silently
never lands in group_members and the group is invisible to them.
Reported from live testing (CLI `member invite grenet`, grenet saw
nothing).

Dependency back to `get_current_user`. The stale test now asserts the
intended behaviour: a node token may add a member to its own
operator's group (201) but not to a group it does not own (403).
Third-review M6 marked WITHDRAWN.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011pG75yGK3NthNfyjH74omG

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