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/files_bc579148-b8ca-4043-be6c-4dfb65d44283.csv

@@ -0,0 +1 @@
+File_No,Id,File_Type,Regarding,Opened,Closed,Empl_Num,Rate_Per_Hour,Status,Footer_Code,Opposing,Hours,Hours_P,Trust_Bal,Trust_Bal_P,Hourly_Fees,Hourly_Fees_P,Flat_Fees,Flat_Fees_P,Disbursements,Disbursements_P,Credit_Bal,Credit_Bal_P,Total_Charges,Total_Charges_P,Amount_Owing,Amount_Owing_P,Transferable,Memo