## Problem
A **free team can invite a paid team's leader** to join. The leader
accepts, and:
- `acceptInvitation` moves them onto the inviting team and removes their
membership from the old team, but only deletes the old team if it's
**personal**.
- Their paid (non-personal) team is left **memberless but still
subscribed** — an orphaned Stripe subscription billing for a team nobody
is in.
## Root cause
Two gaps in `SaasTeamService.acceptInvitation`:
1. The pre-accept guard checks `hasPaidSubscription(acceptingUser)` →
`existsActivePaidSubscriptionForUser(supabaseId)`, keyed on
**`user_id`**. A team's plan is keyed on **`team_id`**
(`existsActiveSubscriptionForTeam`), so a paid team's *leader* isn't
caught and accepts freely.
2. The 'leave existing teams' loop deletes the membership directly and
only marks **personal** teams for deletion — it bypasses the last-leader
protection `leaveTeam` already enforces (`"Cannot leave as the last team
leader. Transfer leadership first."`), and never cleans up / cancels the
non-personal team.
There is no in-app subscription-cancel path (cancellation is
Stripe-portal/webhook driven), so nothing reconciles the orphan after
the fact — it has to be prevented.
## Fix
Add `assertCanLeaveCurrentTeamsToJoinAnother(user)`, called in
`acceptInvitation` before any membership changes. For each non-personal
team where the user is the **last leader**, the accept is rejected:
- team has an active subscription → *"Cancel the plan or transfer
leadership before joining another team."*
- otherwise → *"Transfer leadership before joining another team."*
This mirrors the protection `leaveTeam` already has and is
team/leadership-aware, closing the `user_id`-vs-`team_id` gap. Regular
members and teams with another leader are unaffected.
## Verification
- `ENABLE_SAAS=true ./gradlew :saas:compileJava` — passes.
- Manual: with a paid team leader, accepting an invite to another team
should now be rejected with the message above; verify a non-leader
member can still accept.
## Note
This prevents *new* orphans. Any teams already orphaned by this bug
(memberless, still subscribed) would need a one-off reconciliation —
happy to follow up with a query/cleanup if useful.
