feat(invite): add GET /v1/invites/{code} + rework POST accept as signup+join
- V027 migration: add invited_by FK column on household_invite
- HouseholdInvite entity: add invitedBy field, set on createInvite
- New DTOs: InviteInfoResponse, AcceptInviteRequest
- HouseholdService: add getInviteInfo(), rewrite acceptInvite(code, name, email, password) — creates UserAccount + joins household in one transaction
- HouseholdController: GET /v1/invites/{code} (unauthenticated), POST /v1/invites/{code}/accept creates session after join
- SecurityConfig: permitAll() for /v1/invites/*, sessionFixation().changeSessionId()
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Marcel Raddatz
92f25e56fc