v0.5.4

Fixed

• CodeFirst column upgrades on tables that already have data (#31). SysUser.ForceTotp/TotpEnabled and SysSession.AbsoluteExpiresAt are now nullable database columns — SQL Server rejects ADDing a NOT NULL column without a DEFAULT to a non-empty table, so upgrading a live kernel install used to fail on this dialect. Existing rows read back as NULL; the read path treats that as the pre-upgrade default (MFA flags false, absolute expiry falling back to ExpiresAt) without changing the CLR type of the already-published public properties. Covered by a new regression test, CodeFirstNullableUpgradeTests (drops the columns from a seeded table, then asserts CodeFirst re-adds them and old rows still read correctly), now also run in the SqlServer dialect-sensitive CI subset.
• Documented the underlying rule for anyone adding columns to an already-shipped entity: docs/coding-standards.md §1.3, a new "已有表加列" section in skills/create-entity.md, and a tip box in the deployment guide explaining why evolved columns must be nullable.