refactor: extract Migrator interface and un-skip migration handler tests

[mrofreP]

db/migration_test.go had three tests disabled with skips explaining
they needed a mockable migrate.Migrate or a real database:

• TestMigrationHandler_Step
• TestMigrationHandler_RunMigrations
• TestNewMigrationHandler (empty body, just a placeholder)

The blocker was that MigrationHandler embedded *migrate.Migrate
directly and MigrationStep / ForceVersion called globals.LogAndExit
on both success and failure — so callers could not observe outcomes
and tests could not run in-process.

Refactor:

• db/migration.go

  • Introduce a Migrator interface (Steps, Version, Up, Force) that
    captures the subset of *migrate.Migrate MigrationHandler uses.
  • Replace the *migrate.Migrate embedding with a Migrator field.
  • Add NewMigrationHandlerWith(m Migrator) for test injection.
  • MigrationStep now returns (uint, error); ForceVersion returns
    error. Neither calls LogAndExit — that decision belongs to the
    CLI layer, not the library.
  • RunMigrations still returns error (unchanged shape); implementation
    routes through the new interface field.

• cmd/cservice-api/main.go

  • runMigrations now handles the errors from MigrationStep and
    ForceVersion, calling globals.LogAndExit with the same messages
    that previously lived in the library.

• db/migration_test.go

  • Add a hand-rolled mockMigrator (no mockery / no external mock
    library) that records last-call args and returns configurable
    errors.
  • Un-skip TestMigrationHandler_Step with a table covering up-one,
    down-one, Steps failure (asserts the wrapped error mentions the
    direction), and Version failure.
  • Un-skip TestMigrationHandler_RunMigrations with a table covering
    clean apply, the "no change" success convention, and other-error
    propagation.
  • Add TestMigrationHandler_ForceVersion covering happy path and
    error propagation.
  • Delete the placeholder TestNewMigrationHandler (empty body, only a
    t.Skip — was dead scaffolding, not a test).

Breaking API changes on the exported MigrationHandler:

• MigrationStep(int) → MigrationStep(int) (uint, error)
• ForceVersion(int) → ForceVersion(int) error
  Only main.go consumed these; caller updated in the same commit. The
  project is pre-1.0 (README: "WORK IN PROGRESS ... DO NOT USE IN
  PRODUCTION") so this is an acceptable break.

Net effect: three previously-unreachable code paths in MigrationHandler
now have real unit test coverage without needing a live Postgres.

[codacy-production]

Up to standards ✅

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