#2373 Notify invited users of a collective invite (inbox + email)

Problem

When a user is invited to a collective, nothing tells them. The invitation sits in a pending state and the invited operative has no signal that it exists — they only find out if the inviter tells them out of band. That stalls collective onboarding and makes invites look broken.

Proposed solution

Fan the invite out over the two channels we already have:

1. Swamp Club inbox notification — on invite creation, deliver a notification message to the invited user's swamp.club inbox naming the collective and who invited them, with a link to the collective list page.
2. Email — send the invited user an email covering the same thing, with a link that lands them on the collective list page where the pending invite can be reviewed and accepted or declined.

Both should fire for the same event so the two channels stay consistent.

Notes / open questions

• The invite may be addressed to an email that has no swamp.club account yet. In that case only the email channel applies; the inbox notification should be delivered (or backfilled) once the account exists so the invite is not lost. Related: #1287 (email invitation not linked to the account created from it).
• The collective list page needs to render pending invites with accept/decline affordances for the deep link to be useful.
• Notification delivery should reuse the existing inbox/nurture delivery path rather than adding a second one — see #2021 (deliver to the swamp-club inbox on the same event) and #2024 (swamp inbox), so the invite also shows up for CLI users.

Acceptance criteria

• Inviting a user to a collective creates an inbox notification for that user naming the collective and the inviter.
• The same event sends an email to the invited address with a link to the collective list page.
• Following either link lands on the collective list page with the pending invite visible and actionable (accept / decline).