feat(tier2): multi-host / collaborators — Block C

Events can now have multiple users with distinct roles:
  owner   — manage collaborators, delete event, full access
  editor  — manage guests, tokens, CSV import, patch event
  viewer  — read-only access to everything

Schema (migration 0008)
- collaborator_role ENUM + event_collaborators + collaborator_invites
- Backfill: every existing events.host_id becomes an owner row
- EventRepo.Create seeds the owner row in the same transaction so
  no future event can exist without one

Authz
- New requireRole(eventID, userID, minRole) helper. Non-members 404;
  insufficient role 403. Replaces requireEventOwner across every
  shared-role handler (events.get/update, guests CRUD, tokens issue/
  rotate/bulk, csv preview/commit/template, activity, ws-ticket)
- events.delete + collaborator management stay owner-only
- GET /events lists every event the user has any role on
- /events/{id} response now embeds your_role for UI branching

Collaborator endpoints
- GET    /events/{id}/collaborators           (viewer+)
- POST   /events/{id}/collaborators           (owner)  — sends invite email
- PATCH  /events/{id}/collaborators/{user_id} (owner)  — role change
- DELETE /events/{id}/collaborators/{user_id} (owner)  — refuses last owner
- DELETE /events/{id}/collaborators/pending   (owner)  — cancel invite
- GET    /invites/{token}                     (public) — preview summary
- POST   /invites/{token}/accept              (authed) — atomic accept

Invitations
- SHA-256 hashed in DB; raw value only lives in the email link
- 7-day TTL, single-use, email-bound (caller's email must match)
- New SendCollaboratorInvite on auth.EmailSender + Resend/SMTP/SES
  senders + log stub; collaborator_invite.html/txt branded template

Frontend
- TeamCard.vue on the event detail page: lists collaborators with
  inline role-change + remove, pending-invites with cancel, invite
  modal (email + role). Owner-only actions hidden for editors/viewers
- /invites/[token] accept page: shows invite summary, prompts signup
  or sign-in with pre-filled email, refuses mismatched accounts

Tests (all 6 pass on the existing testcontainers harness)
- backfill: legacy host gets owner role
- role enforcement: viewer can read, editor can write guests but not
  delete/manage team, non-member 404s everywhere
- last-owner removal refused (400)
- shared events show up in collaborator's /events list
- invite flow: create → preview → accept → role granted → replay 410
- email mismatch on accept returns 403
- expired invite returns 410

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/api/guests.go

@@ -12,6 +12,7 @@ import (
 type guestHandler struct {
 	guests   *storage.GuestRepo
 	events   *storage.EventRepo
+	collabs  *storage.CollaboratorRepo
 	enforcer *tierEnforcer
 }
 
@@ -33,7 +34,7 @@ func (h *guestHandler) create(w http.ResponseWriter, r *http.Request) {
 	if !ok {
 		return
 	}
-	if _, ok := requireEventOwner(w, r, h.events, eventID, hostID); !ok {
+	if _, _, ok := requireRole(w, r, h.events, h.collabs, eventID, hostID, domain.RoleEditor); !ok {
 		return
 	}
 	if !h.enforcer.allowGuestCreate(w, r, hostID, eventID) {
@@ -93,7 +94,7 @@ func (h *guestHandler) update(w http.ResponseWriter, r *http.Request) {
 	if !ok {
 		return
 	}
-	if _, ok := requireEventOwner(w, r, h.events, eventID, hostID); !ok {
+	if _, _, ok := requireRole(w, r, h.events, h.collabs, eventID, hostID, domain.RoleEditor); !ok {
 		return
 	}
 
@@ -143,7 +144,7 @@ func (h *guestHandler) delete(w http.ResponseWriter, r *http.Request) {
 	if !ok {
 		return
 	}
-	if _, ok := requireEventOwner(w, r, h.events, eventID, hostID); !ok {
+	if _, _, ok := requireRole(w, r, h.events, h.collabs, eventID, hostID, domain.RoleEditor); !ok {
 		return
 	}
 	if err := h.guests.Delete(r.Context(), eventID, guestID); err != nil {
@@ -166,7 +167,9 @@ func (h *guestHandler) list(w http.ResponseWriter, r *http.Request) {
 	if !ok {
 		return
 	}
-	if _, ok := requireEventOwner(w, r, h.events, eventID, hostID); !ok {
+	// Listing guests is viewer+. Editing is gated separately at the
+	// PATCH/POST/DELETE call sites.
+	if _, _, ok := requireRole(w, r, h.events, h.collabs, eventID, hostID, domain.RoleViewer); !ok {
 		return
 	}