Recordings, the recording list/delete routes, the trainer's dataset
loader and model export, the models routes and the adaptive classifier
all used paths relative to the process cwd (`data/recordings`,
`data/models`, `data/adaptive_model.json`), so `--data-dir` only covered
config and the session secret.
Add `recordings_dir(data_dir)` and `models_dir(data_dir)` and use them
everywhere. `MODELS_DIR` still overrides the models directory, and the
trainer now honours it too. With the default `--data-dir data` the paths
are unchanged.
Fixes#2088
Co-Authored-By: claude-flow <ruv@ruv.net>
