Fix PHONE.csv import duplicate constraint error

- Implement upsert logic in import_phone() function
- Check for existing (id, phone) combinations before insert
- Track duplicates within CSV to skip gracefully
- Update existing records instead of failing on duplicates
- Add detailed statistics: inserted, updated, skipped counts
- Align with upsert pattern used in other import functions
- Add documentation in docs/PHONE_IMPORT_FIX.md

Fixes: UNIQUE constraint failed: phone.id, phone.phone error
when re-importing or uploading CSV with duplicate entries

data-import/unknown_e6f04b8b-cad9-4b0a-82d5-4ee04459a85f.csv

@@ -0,0 +1 @@
+File_No,Id,Footer_Code,Date,Item_No,Empl_Num,T_Code,T_Type,T_Type_L,Quantity,Rate,Amount,Billed,Note