fix(auth): initialize bootstrap admin as employee

[higenglele]

The bootstrap admin receives the super_admin role but inherits user_type=user, so dashboard authentication rejects it. Initialize it as employee and add migration 10 to repair existing installations where migration 2 has already succeeded.

The repair only updates the active, non-deleted default admin with an active super_admin role and user_type=user. It preserves passwords, account status and role assignments, and is idempotent. This uses the existing enums and migration framework.

Fixes #36.

Validation:

• Reproduced the incorrect bootstrap type with a failing regression test before applying the fix.
• Passed bootstrap creation and repair tests, including ordinary users, missing roles, disabled/deleted accounts and repeated execution.
• Passed the existing backend CI test suite plus ./internal/migration/... with Go 1.26 and -tags dev.
• Ran gofmt and git diff --check.

The migration regression tests use SQLite. A MySQL integration run was not performed.