Move reverse-migration logic from get_model_folder() (hot path, called on
every metadata/example-images request) to ExampleImagesMigration, where it
runs once at startup. On network storage this was causing 22-38s delays
per LoRA card click.
Additionally optimize prune_stale_example_images() to read the directory
listing once instead of per image entry (O(N*M) → O(M)). Also reorder
consolidation checks so regex filters run before filesystem stat calls.
