[DB] Migrations-apply helper (no DSL, no schema ownership)

[cssbruno]

Add a migrations-apply helper for user-authored SQL files.

Scope:

• Apply .sql files in order through database/sql.
• Do not introduce a migration DSL.
• Do not make GOWDK own application schema design.

Acceptance criteria:

• Helper applies ordered SQL files idempotently or documents the exact tracking contract.
• Errors include file names and step information.
• Docs show a minimal usage example.