Add comprehensive DB interview guide with deep ALTER TABLE section

[WsWiss]

Summary

• add a new interview prep document for databases: DB_INTERVIEW_GUIDE_ALTER_TABLE.md
• cover core DB fundamentals (keys, indexes, normalization, transactions, joins)
• provide an expanded practical section on ALTER TABLE:
  • common operations and syntax examples
  • lock/transaction/rewrite implications
  • PostgreSQL and MySQL operational nuances
  • zero-downtime migration patterns
  • interview-ready answer templates and self-check questions

Testing

• documentation-only change; no runtime tests required