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

backend/training.py

@@ -155,11 +155,11 @@ def promote(model_id: int) -> dict:
     project = projects.get(version["project_id"])
     base_path = os.path.join(config.project_dir(project["slug"]), "base", "model.pt")
     os.makedirs(os.path.dirname(base_path), exist_ok=True)
-    shutil.copyfile(version["weights_path"], base_path)
+    shutil.copyfile(config.resolve_data_path(version["weights_path"]), base_path)
     with db.cursor() as cur:
         cur.execute(
             "UPDATE projects SET base_model_path = ?, base_model_kind = 'trained' WHERE id = ?",
-            (base_path, project["id"]),
+            (config.rel_data_path(base_path), project["id"]),
         )
     return projects.get(project["id"])
 
@@ -289,7 +289,9 @@ def _run_train(job) -> None:
                                                parent_model_path, metrics, base_metrics,
                                                created_at, augment, name)
                    VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)""",
-                (project["id"], version, weights, project["base_model_path"],
+                (project["id"], version, config.rel_data_path(weights),
+                 config.rel_data_path(project["base_model_path"])
+                 if project["base_model_path"] else project["base_model_path"],
                  json.dumps(comparison["new"]), json.dumps(comparison["base"]), time.time(),
                  json.dumps(augmentation["settings"]),
                  _generate_model_name(project, start_point, settings["epochs"], class_ids)),