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_9a098ad0-e280-44bc-9175-fa7dd85553ea.csv

@@ -0,0 +1,2 @@
+Appl_Title,L_Head1,L_Head2,L_Head3,L_Head4,L_Head5,L_Head6,L_Head7,L_Head8,L_Head9,L_Head10,Default_Printer
+"DELPHI CONSULTING GROUP, INC",,,,,,,,,,,5