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_379e12b9-f7f0-4c07-a80b-3d92f84ffb80.csv

@@ -0,0 +1,6 @@
+Number,Name,Port,Page_Break,Setup_St,Phone_Book,Rolodex_Info,Envelope,File_Cabinet,Accounts,Statements,Calendar,Reset_St,B_Underline,E_Underline,B_Bold,E_Bold
+1,HP Laser Jet 4L,LPT1,FormFeed,,,,\027&l81a3h1O\027(s1p12.5v0s0b4101T,,,,,\027E,\027&d3D,\027&d@,\027(s3B,\027(s0B
+2,HP Laserjet III,LPT1,FormFeed,,,,\027&l81a3h1O\027(s1p12.5v0s0b4101T,,,,,\027E,\027&d3D,\027&d@,\027(s3B,\027(s0B
+3,Cannon Bubble Jet 10ex,LPT1,FormFeed,,,,,,,,,,\027-1,\027-0,\027G,\027H
+4,Okidata OL 400,LPT1,FormFeed,,,,\027&l81a3h1O\027(s1p12.5v0s0b4101T,,,,,\027E,\027&d3D,\027&d@,\027(s3B,\027(s1B
+5,HP Laser Jet 5M,LPT1,FormFeed,,,,\027&l81a3h1O\027(s1p11v0s0b4148T,,,,,\027E,\027&d3D,\027&d@,\027(s3B,\027(s0B