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/api/models.py

@@ -7,7 +7,7 @@ from fastapi import APIRouter, HTTPException
 from fastapi.responses import FileResponse
 from pydantic import BaseModel
 
-from backend import hardware, training
+from backend import config, hardware, training
 from backend.api.common import project_or_404
 
 router = APIRouter(tags=["models"])
@@ -55,10 +55,11 @@ def list_models(project_id: int) -> dict:
 @router.get("/api/models/{model_id}/weights")
 def download_weights(model_id: int):
     version = training.get_version(model_id)
-    if version is None or not os.path.isfile(version["weights_path"]):
+    weights = config.resolve_data_path(version["weights_path"]) if version else None
+    if version is None or not os.path.isfile(weights):
         raise HTTPException(404, "No weights for that version")
     name = version.get('name') or f"v{version['version']}"
-    return FileResponse(version["weights_path"], media_type="application/octet-stream",
+    return FileResponse(weights, media_type="application/octet-stream",
                         filename=f"{name}-best.pt")