[Bug] Short-code generation TOCTOU — retry on unique-key collision

[Salem874]

Problem

sp_generateShortCode is check-then-insert with no retry on a UQ_shortcode_org collision; concurrent creates can surface a generic failure instead of producing a fresh code.

Affected

• web/_sql/procedures/sp_generateShortCode.sql
• web/Go2My.Link/_functions/shorturl_create.php (createShortURL())

Fix

Make creation atomic, or catch mysqli errno 1062 in createShortURL() and retry sp_generateShortCode a bounded number of times.

Acceptance criteria

• Concurrent create requests never fail due to a code collision (bounded retry).

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