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/employee_e4f6f32a-0808-4c82-b5ff-0defd742c33e.csv

@@ -0,0 +1,7 @@
+Empl_Num,Empl_Id,Rate_Per_Hour
+0,UNASSIGNED,200.00
+1,MUELLER\JS,200.00
+2,HABICH\RA,200.00
+3,GUETTEL\SR,200.00
+4,BENDT\D,200.00
+5,BENDT\LF,200.00