AssetManager 0.3.8.0 - Central field catalog and MeshCentral GUI configuration

IMPORTANT:
- Stable rollback version before this structural migration: 0.3.7.10
- Create a PostgreSQL/database backup before first start.
- Do not copy .env, config.json, data, uploads or logs from this ZIP.

New database objects:
- field_definitions
- asset_field_values
- mesh_field_mappings (prepared for future database-backed mappings)
- category_fields.field_definition_id

Design:
- System fields remain physical columns in assets.
- User-created fields are type-aware values in asset_field_values. No ALTER TABLE assets is needed.
- Categories only control active/required/show-in-list/sort order.
- Labels, descriptions, types and read-only properties are central.
- MeshCentral connection, sync behavior, update rules and source-path mappings are editable in the web UI.