fix: migrate legacy account issuers

[korone00]

Summary

• backfill Better Auth 1.7 account issuers before generated migrations
• validate credential ownership and identity collisions in one PostgreSQL transaction
• preserve rolling upgrades with a fail-closed compatibility trigger
• document the Microsoft and unknown-provider manual boundary

Verification

• npm run build
• npm run test (321 passed)
• npm run lint
• isolated PostgreSQL 16 success, rollback, and idempotency integration
• production Bun Alpine migration image
• Verdaccio faithful Outer: 40 passed, 1 conditional skip, 0 failed