Add DB config validation for required username on postgres/mysql

[Copilot]

Databases like PostgreSQL and MySQL require a username for connection. Previously, an empty username would result in a cryptic connection error. This adds upfront validation with a clear error message.

Changes

• Added Validate() method to gorm.Config that checks driver-specific requirements
• postgres and mysql drivers now require non-empty username
• sqlite is exempt (no authentication)
• Validation runs in NewDB() before connection attempt

cfg := gorm.Config{
    Driver: "postgres",
    Host:   "localhost",
    // Username: "", // empty - will now panic with "username is required for postgres driver"
}
db := gorm.NewDB(cfg)

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