[Bug] Data migration: guard zero-date/NULL legacy dates under STRICT sql_mode

[Salem874]

Problem

Migrations copy legacy date columns straight into NOT NULL createdAt/updatedAt under STRICT sql_mode. A single 0000-00-00/NULL legacy row aborts the whole transactional batch — risking the live 480-URL / 5-org / 7-user migration.

Affected

• web/_sql/migrations/001_migrate_organisations.sql, 002_migrate_users.sql, 003_migrate_categories.sql, 004_migrate_shorturls.sql

Fix

Guard every legacy date, e.g. IFNULL(NULLIF(old.dateCol,'0000-00-00 00:00:00'), NOW()); or pre-clean source dates. Add a migrated-vs-source row-count assertion after COMMIT.

Acceptance criteria

• Dry-run migration succeeds against a copy of legacy data containing zero/NULL dates.
• Post-migration row counts match source counts.

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