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/footers_4c60ac53-ac80-41b5-94e7-2bedc0d90806.csv

@@ -0,0 +1,6 @@
+F_Code,F_Footer
+1,TERMS: BALANCE DUE UPON RECEIPT - THANK YOU
+2,THIS ACCOUNT IS PAST DUE.  PLEASE PAY THE BALANCE DUE IMMEDIATELY!
+3,PAYMENT IN FULL REQUIRED PRIOR TO RELEASING WORK PRODUCT
+4,PAYMENT IN FULL REQUIRED PRIOR TO RELEASING WORK PRODUCT
+5,EACH PARTY RESPONSIBLE FOR ONE HALF OF TOTAL FEE