Promote BOM source from metadata JSONB to a dedicated VARCHAR(20)
column with CHECK constraint ('manual' or 'assembly').
- Add migration 012_bom_source.sql (column, data migration, cleanup)
- Add Source field to Relationship and BOMEntry structs
- Update all SQL queries (GetBOM, GetWhereUsed, GetExpandedBOM, Create)
- Update API response/request types with source field
- Update CSV/ODS export to read e.Source instead of metadata
- Update CSV import to set source on relationship directly
- Update frontend types and BOMTab to use top-level source field
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