feat(db): add source column to relationships table (#44)

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

migrations/012_bom_source.sql

@@ -0,0 +1,16 @@
+-- Add source column to relationships table to distinguish assembly-derived
+-- BOM entries from manually-added ones.
+ALTER TABLE relationships
+    ADD COLUMN source VARCHAR(20) NOT NULL DEFAULT 'manual'
+    CHECK (source IN ('manual', 'assembly'));
+
+-- Migrate existing metadata.source values where they exist.
+-- The metadata field stores source as a free-form string; promote to column.
+UPDATE relationships
+SET source = 'manual'
+WHERE metadata->>'source' IS NOT NULL;
+
+-- Remove the source key from metadata since it's now a dedicated column.
+UPDATE relationships
+SET metadata = metadata - 'source'
+WHERE metadata ? 'source';