fix(upload): store relative paths in uploaded_files.storage_path

[tanbro]

Abandoned

• see fix(upload): store relative paths in uploaded_files.storage_path #235

Previously, The uploaded_files table's storage_path field stored absolute file paths in the database, which causes issues when:

• The database / data files is migrated to a different server
• XAGENT_UPLOADS_DIR environment variable is changed
• Backing up and restoring to a different location

Changes:

• Add web.utils.file module with path conversion functions:
  • to_relative_path(): convert absolute to relative (without user_id prefix)
  • to_absolute_path(): convert relative to absolute
  • find_file_by_path(): query files with absolute/relative path compatibility
• Add UploadedFile.absolute_path property that handles both absolute and relative paths for backward compatibility
• Update all file creation locations to store relative paths:
  • web/api/files.py: file upload API
  • web/api/websocket.py: task output files (all locations)
  • web/api/kb.py: knowledge base file upload
  • core/workspace.py: agent workspace file registration
  • scripts/backfill_uploaded_files.py: backfill script
• Update all file path queries to check both absolute and relative formats
• Add migrate_uploads_file_abs_path.py script for migrating existing data
  • Only migrates paths that match current UPLOADS_DIR configuration
  • Safe to run multiple times (e.g., after changing XAGENT_UPLOADS_DIR)
• Add tests to verify relative path storage for both regular upload and KB upload

Verified: New files created via agent now store relative paths like "web_task_37/output/fibonacci_polar_final.png" instead of absolute paths. Old records with absolute paths continue to work via backward compatibility logic.

[tanbro]

The PR is abandoned, see #235