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_94d488d7-6140-4881-acde-7ecbefe46e70.csv

@@ -0,0 +1,53 @@
+Abrev,St
+AK,Alaska
+AL,Alabama
+AR,Arkansas
+AZ,Arizona
+CA,California
+CO,Colorado
+CT,Connecticut
+DC,DC
+DE,Delaware
+FL,Florida
+GA,Georgia
+HI,Hawaii
+IA,Iowa
+ID,Idaho
+IL,Illinois
+IN,Indiana
+KS,Kansas
+KY,Kentucky
+LA,Louisiana
+MA,Massachusetts
+MD,Maryland
+ME,Maine
+MI,Michigan
+MN,Minnesota
+MO,Missouri
+MS,Mississippi
+MT,Montana
+NC,North Carolina
+ND,North Dakota
+NE,Nebraska
+NH,New Hampshire
+NJ,New Jersey
+NM,New Mexico
+NV,Nevada
+NY,New York
+OH,Ohio
+OK,Oklahoma
+OR,Oregon
+PA,Pennsylvania
+PR,Puerto Rico
+RI,Rhode Island
+SC,South Carolina
+SD,South Dakota
+TN,Tennessee
+TX,Texas
+UT,Utah
+VA,Virginia
+VT,Vermont
+WA,Washington
+WI,Wisconsin
+WV,West Virginia
+WY,Wyoming