modules/admin: split migration into workers and orchestrator

- migration-workers.server.ts: Pure worker functions (migrateMembers,
  migrateArticles, migrateProjects, migrateImages) with Logger param
- migration.server.ts: Orchestration (clone repo, run workers, manage state)

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: aster-void