Backend startup emits many EF `fail` schema logs even when health is ok

[simplerjiang]

• Found by: backend/API test agent and source UI smoke.
• Impact: Startup logs look failed while /api/health is ok, which can hide real failures.
• Repro:
  1. Run source backend.
  2. Request /api/health.
  3. Inspect startup stdout.
• Expected: Idempotent schema initialization should not log expected existing-column cases as fail.
• Actual: Many fail: Microsoft.EntityFrameworkCore.Database.Command[20102] entries for ALTER TABLE ... ADD COLUMN ....
• Claim: Assign backend/database owner.
• Repair: Check column existence before ALTER or downgrade expected duplicate-column handling.
• Verification: Source startup log has no expected fail schema noise; health remains ok.