feat: add project base path and template path handling in report generation

- Introduced PROJECT_BASE_PATH constant for consistent project directory reference.
- Updated Reports class to require template_path for loading HTML templates.
- Modified DataManagerRepository to pass the new template_path when generating reports.

model_manager/runtime_paths.py

@@ -9,6 +9,9 @@ RUNTIME_DATA_ROOT = '/var/lib/model-manager'
 # Training reports (HTML, CSV exports, etc.) and related outputs.
 REPORTS_ROOT = join(RUNTIME_DATA_ROOT, 'reports')
 
+# project base path
+PROJECT_BASE_PATH = '/app/model_manager'
+
 # Per-training run folders (name + timestamp); cleanup cron deletes stale entries here.
 REPORTS_TEMP_DIR = join(REPORTS_ROOT, 'temp')