fix: suppress redundant UNIQUE on primary key fields in SQL exports

[20syldev]

Summary

• PRIMARY KEY already implies uniqueness in all SQL databases (MySQL, MariaDB, PostgreSQL, SQLite, MSSQL, Oracle)
• When a field had both primary: true and unique: true, the export was emitting UNIQUE on the column definition and a PRIMARY KEY(...) clause at the end of the CREATE TABLE, resulting in a redundant duplicate index
• Fix: suppress the UNIQUE keyword when the field is already marked as primary in all SQL exporters (mysql.js, mariadb.js, postgres.js, mssql.js, sqlite.js, oraclesql.js, generic.js)

Root cause

Template files (template1.js through template6.js) set unique: true on primary key fields. The SQL export logic didn't account for this redundancy.

Test plan

• Create a table in MySQL/MariaDB mode with a primary key
• Export to SQL and verify the primary key column does not have UNIQUE in its column definition
• Verify the PRIMARY KEY(...) clause is still present at the end of CREATE TABLE
• Test with PostgreSQL, SQLite, MSSQL, and Oracle exports

Fixes #961

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