fix(admin): delete organization instances

[efegokdemir]

What this changes

Delete an operator-managed organization together with its stored instance connections. Instance rows were only soft-deleted before the organization delete, so SQLite foreign-key enforcement rejected the transaction and retained the organization data.

Why

The instances.org_id foreign key has no cascade, and instance_events.instance_id also requires its rows to be removed first. The deletion helper now removes instance events, instances (including stored admin tokens), invitations, and memberships in one transaction before deleting the organization.

Testing

• npm test — passed (230 tests)
• npm run check — passed with 0 errors and 10 pre-existing warnings
• LAUNCHPAD_JWT_SECRET=test npm run build — passed
• git diff --check — passed

Closes #61

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[efegokdemir]

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