AssetManager 0.3.8.1 - MeshCentral field mapping correction

Changes:
- mesh_field_mappings is now the database-backed source of truth.
- Existing config.json field mappings are imported once into the database.
- Default MeshCentral source paths are seeded automatically when missing.
- Added update_rule column through the lightweight migration.
- Multiple-value mode, separator, enabled state and update rule are persisted.
- Synchronization reads enabled mappings directly from PostgreSQL.
- Browser now reports non-JSON backend responses instead of only JSON.parse errors.

Important:
- .env, config.json, database data, uploads and logs are not included.
- The rollback version before this structural change remains 0.3.7.10.
- Create a PostgreSQL backup before installation.