fix: resolve model weight paths against DATA_DIR (REQ-187)

- config.py: resolve_data_path (legacy abs + rel) + rel_data_path
- all file-opening reads wrapped: preview, autolabel, training, model
  download, live count, projects.get; training_start_point hack replaced
- new writes store paths relative to data/
- legacy stale rows (/home/asus/reTraining/...) resolve without migration
- requirements: REQ-187 added; REQ-188 (per-class max box) + REQ-186
  copy-line amendment drafted for the next task

docs/design.md

@@ -19,7 +19,11 @@ FastAPI  ──►  jobs (1 worker thread, 1 GPU)
 
 Storage split rule: **SQLite holds metadata and status; the disk holds pixels, final labels,
 and weights.** The master dataset must stay useful even if the database is lost
-(REQ-006, REQ-054).
+(REQ-006, REQ-054). Model weight paths stored in SQLite (`projects.base_model_path`,
+`model_versions.weights_path`, `model_versions.parent_model_path`) are written **relative
+to `data/`** and resolved against `DATA_DIR` on every read (REQ-187), so a moved data
+folder keeps every trained model reachable; legacy absolute rows from older installs go
+through the same resolver.
 
 ## Disk layout (REQ-006)