[Bug] Org invitations: re-invite blocked by UQ_org_email_pending including status

[Salem874]

Problem

tblOrgInvitations.UQ_org_email_pending (orgHandle, email, status) includes status, so only one row per lifecycle state is allowed. A second cancel/expire for the same org+email collides with the unique key, so re-invite cycles fail.

Affected

• web/_sql/schema/014_org_invitations.sql
• web/_functions/org.php (invite/cancel/expire flow)

Fix

Enforce "one PENDING per org+email" only — e.g. a generated column pendingKey = IF(status='pending', CONCAT(orgHandle,'|',email), NULL) with a UNIQUE key (InnoDB exempts NULLs), and drop status from the guarantee.

Acceptance criteria

• Cancel → re-invite → cancel works repeatedly for the same org+email.
• Still cannot have two simultaneous PENDING invites for the same org+email.

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From the 2026-06-04 schema review — see docs/SCHEMA_REVIEW_2026-06-04.md.