feat: invite link system for trust graph onboarding

[pkarlsson-codes]

Summary

Allow participants to generate shareable invite links that bring new users into the system and seed the trust graph with real-world relationships.

Link mechanics

• Multi-use — one link can be followed by any number of people
• No expiry by default
• Revocable — the inviter can cancel a link at any time and generate a new one
• Multiple links — a participant can maintain several active links simultaneously, each with an optional personal note visible only to themselves (e.g. "meetup March 2026", "book club")

Delivery

• The system generates the link; the inviter sends it themselves via whatever channel they prefer (Signal, email, in person, etc.)
• No system-mediated email/SMS delivery — the system never handles contact details

Registration flow

1. Invitee follows the link
2. If not yet registered, they are prompted to sign up
3. Once registered, invitee is shown: "[Pseudonym] invited you — do you want to extend trust to them?"
4. Inviter is notified: "[Pseudonym] accepted your invite — do you want to extend trust to them?"
5. Neither direction is automatic — both participants decide independently

What this solves

• Primary discovery mechanism for people who have real-world relationships before joining the platform
• Seeds the trust graph from day one — every new user arrives with at least the opportunity for a trust relationship
• Handles the cold-start problem naturally: the invite tree grows the graph as the platform grows

Bounded context

Reputation — trust relationships are owned here. The invite link itself may need a small slice in a shared or identity context for token generation and lookup.