fix: remove dead SchemaMigrationService and unused Migration entity

[dedukpe]

Summary

Remove SchemaMigrationService, the Migration entity, and the entities/ directory — all dead code that was never wired up.

Closes

Closes #1199

Changes

File	Action
src/migrations/schema-migration.service.ts	Deleted
src/migrations/entities/migration.entity.ts	Deleted
src/migrations/entities/	Deleted (directory)
src/config/datasource.ts	Updated comments to reflect the removal

Why

SchemaMigrationService queries and inserts into a schema_migrations table, but:

1. No module registers it — it is never injected anywhere in the codebase.
2. No migration creates the table — the project uses TypeORM's built-in migrations table.
3. No code imports the Migration entity — zero references across the entire src/ directory.

The service and entity only sat in the src/migrations/ directory, where the TypeORM migration glob in datasource.ts had to be explicitly configured to ignore them (via the [0-9]* timestamp prefix pattern). This makes the code misleading and a maintenance burden.

Verification

• tsc --noEmit passes with no errors
• nest build succeeds
• No imports or references to the deleted files remain in the codebase

[RUKAYAT-CODER]

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