The sequence counter (sequences_by_name table) can get out of sync
with the items table if items were seeded/imported directly or if a
previous create failed after incrementing the sequence but before
the insert committed. This causes Generate() to return a part number
that already exists, hitting the unique constraint on items.part_number.
Add a retry loop (up to 5 attempts) in HandleCreateItem that detects
PostgreSQL unique violation errors (SQLSTATE 23505) via pgconn.PgError
and retries with the next sequence value. Non-duplicate errors still
fail immediately. If all retries are exhausted, returns 409 Conflict
instead of 500.
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