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/rvarlkup_74f19c2a-bd39-40b6-8ea3-df88976b029f.csv

@@ -0,0 +1,26 @@
+Identifier,Query
+AGE,Age (31)?
+COUNTY_OF_RES,County of residence/practice?
+EDUCATION,Highest grade completed?
+EMPLOYED_AS,Occupation?
+EMPLOYER_A1,Employer's street address - 1st line?
+EMPLOYER_A2,Employer's street address - 2nd line?
+EMPLOYER_CSZ,"Employer's city, state and zip code?"
+EMPLOYER_NAME,Name of Employer?
+FIRM_NAME,Name of law office?
+GENDER,"Gender (Male, Female)?"
+HE/SHE,He or she for subject?
+HIM/HER,Him or her for subject?
+HIS/HER,His or her for subject?
+INITS,Initials (WJS)?
+LCEP,Life Cycle Earnings Pattern Age Category?
+LE,Life expectancy (22.4)?
+NOT_EXPIRES,"Notary's comm. (is permanent,expires on ##/##/##)?"
+NOT_NAME,Notary's full name (Bob E. Smith)?
+NOT_ST,Notary's state of commission (Wisconsin)?
+POSS_FORM,"Possessive form of subject's last name (','s)?"
+RACE,Subject's race (White)?
+SB#,State Bar Number?
+SEC_INITS,Secretary's Initials (ih)?
+SEC_NAME,Secretary's full name (Irene Holly)?
+WLE,Worklife expectancy (13.5)?