Add BOM merge endpoint for syncing assembly-derived BOM entries from
FreeCAD's silo-mod plugin.
Merge rules:
- Added: entries in request but not in server BOM are auto-created
with source='assembly'
- Quantity changed: existing entries with different quantity are
auto-updated
- Unchanged: same part and quantity are skipped
- Unreferenced: assembly-sourced entries in server BOM but not in
request are flagged as warnings (never auto-deleted)
- Manual entries are silently ignored in unreferenced detection
Also emits SSE 'bom.merged' event on successful merge (#46).
