Organisation-wide enrolment and teammate visibility #42

Problem

Needly already models organisation membership (Needly.Domain/InstallationMember.cs, GitHubOrganizationMembershipService), so a colleague of an existing user can in principle sign in and get a working Inbox. Nothing in the product tells anybody that.

One admin installing the App should seed an entire organisation. Today it effectively seeds one account, which wastes the single highest-leverage moment in the funnel.

Scope

• After an installation is linked, detect organisation members who have not yet signed in
• Show the installing admin a list and let them invite in one click
• Show every user "9 of your teammates use Needly" with avatars
• A shareable organisation join link that skips App installation entirely, since the App is already installed
• Optional welcome email or Slack message on first enrolment

Acceptance criteria

• Teammate count and avatars visible in Settings and in onboarding
• Invitations are explicit and rate limited; Needly never emails an organisation's members unprompted
• A join link works for anybody who is a verified member of the installed organisation
• A newly joined user lands on a populated Inbox, not an empty state, using historical bootstrap
• Organisation membership is re-verified rather than trusted from a stale record
• Admins can disable auto-enrolment for their organisation

Notes

This is the highest-leverage growth change that requires no new permissions and no new surface. The plumbing exists; only the product surface is missing.