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/notification/templates/collaborator_invite.html

@@ -0,0 +1,16 @@
+{{define "body"}}
+<p style="font-size:13px;letter-spacing:0.2em;text-transform:uppercase;color:#16a34a;margin:0 0 16px;">✦ Team invitation</p>
+<h1 style="font-size:24px;margin:0 0 4px;color:#0a0a0a;">{{.EventName}}</h1>
+<p style="margin:16px 0 20px;">
+  {{.InviterName}} invited you to collaborate on this event as
+  <strong>{{.Role}}</strong>.
+</p>
+<p style="margin:0 0 24px;text-align:center;">
+  <a href="{{.Link}}" style="background:#22c55e;color:#0a0a0a;padding:12px 22px;border-radius:8px;font-weight:600;text-decoration:none;display:inline-block;">Accept invitation</a>
+</p>
+<p style="margin:0 0 8px;color:#64748b;font-size:13px;">
+  This invitation expires in 7 days. If you don't have a GuestGuard account
+  yet, you'll be able to create one before accepting.
+</p>
+<p style="margin:8px 0 0;word-break:break-all;font-size:12px;color:#0f172a;">{{.Link}}</p>
+{{end}}