feat(db): track and roll back code-based migrations

[GaspardKirira]

Summary

Turn MigrationsRunner into a tracked and reversible code-migration runner.

Current behavior

MigrationsRunner stores raw migration pointers and executes every registered migration through up().

It does not:

• inspect migration identifiers;
• persist applied state;
• skip applied migrations;
• validate duplicate identifiers;
• execute down();
• support rollback.

Expected behavior

Code-based migrations must provide deterministic ordering and persistent applied-state tracking similar to file migrations.

Scope

• Validate migration identifiers.
• Sort or explicitly preserve documented ordering.
• Create or reuse a migration tracking table.
• Skip migrations already applied.
• Apply pending migrations transactionally.
• Add rollback support.
• Detect duplicate migration identifiers.
• Define ownership expectations for registered migration objects.
• Add SQLite integration tests.
• Preserve current migration class signatures where possible.

Suggested files

• modules/db/include/vix/db/mig/MigrationsRunner.hpp
• modules/db/src/mig/MigrationsRunner.cpp
• modules/db/tests/code_migrations_runner_test.cpp

Acceptance criteria

• Applied migrations are persisted.
• Applied migrations are not executed twice.
• Duplicate IDs are rejected.
• Rollback invokes the correct down() method.
• Failed migrations are not marked as applied.
• Migration ordering is documented and tested.
• Existing migration implementations remain source-compatible where possible.

Non-goals

This issue does not include:

• automatic C++ migration generation;
• ORM model migrations;
• distributed migration coordination;
• schema introspection.

Suggested labels

• scope:db
• type:feature
• tests
• help wanted
• status:accepted