Epic: invite existing volunteers to register as users (role: volunteer)

[arturasmckwcz]

Why

We have a backlog of registered volunteers — rows in the volunteer table,
created via the public "Become a Volunteer" form or coordinator import — who
have never created a login (User account). They can't sign in, see their
own profile, or get matched through the app. We want to invite all of them by
email to finish registering as a User with role: VOLUNTEER.

Today Volunteer has no direct link to User at all — the only path is
Volunteer.person → Person.users, and that link is frequently empty for
exactly this population.

Design decisions (already made)

• Account creation timing: eager. The User row is created immediately
  when the invite is sent (isActive: false, placeholder password), not
  deferred until the person accepts. No schema/migration changes anticipated.
• Feature scope: one-off backfill only. No persisted coordinator-facing
  "invite" button or per-volunteer endpoint — just a script that clears the
  current backlog. The invite-acceptance flow itself (JWT + accept-invite
  endpoint + FE page) is still a real, reusable piece of infrastructure; only
  the triggering side is one-off for now.

Sub-issues

sdk

• Accept-invite request/response contract — feat: accept-invite request/response contract for volunteer account linking sdk#212

be

• Invite issuance + accept-invite auth flow — feat: invite flow — mint invite tokens for volunteers, accept-invite endpoint #920
• One-off script: invite existing volunteers without a user account — chore: one-off script — invite existing volunteers without a user account #921
• Follow-up: POST /user must link to an existing Person by email instead of creating a duplicate — fix: POST /user must link to an existing Person by email instead of creating a duplicate #923 (found while reviewing feat: volunteer self-registration with email confirmation fe#955; without this, new self-registrations keep recreating the same problem this epic backfills away)

fe

• Invite-acceptance page (set password, activate account) — feat: invite-acceptance page — set password to activate an invited volunteer account fe#958

Acceptance criteria

• Every Volunteer row with a usable person email and no linked User
  has received an invite email
• Invited volunteers can set a password and log in as role: VOLUNTEER
• No duplicate accounts created for volunteers who already have a User
• No database migration required by this epic (or, if one turned out
  necessary, it's called out explicitly with the reason)