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/preview.py

@@ -11,7 +11,7 @@ whatever happens to sit at those coordinates there.
 import os
 from typing import List, Optional
 
-from backend import batches, db, labeling, projects, review
+from backend import batches, config, db, labeling, projects, review
 from backend.autolabel import DEFAULT_IOU, DEFAULT_THRESHOLD, _geometries, _parse_class_params
 
 def preview_frame(
@@ -64,8 +64,10 @@ def preview_frame(
             with db.cursor() as cur:
                 cur.execute("SELECT weights_path FROM model_versions WHERE project_id = ? ORDER BY version DESC LIMIT 1", (project["id"],))
                 row = cur.fetchone()
-                if row and os.path.isfile(row[0]):
-                    m_path = row[0]
+                if row:
+                    resolved = config.resolve_data_path(row[0])
+                    if os.path.isfile(resolved):
+                        m_path = resolved
         yolo_model = YOLO(m_path)
 
     name_to_class_id = {item["name"].strip().lower(): item["class_id"] for item in project["classes"]}