Implement configurable PasswordIdentity schema factory

[dpickett]

Summary

Create a flexible schema factory that generates Zod schemas for password-based authentication with configurable identifier types.

Implementation

• Support email, username, and phone as identifier options
• Compile-time type safety with as const arrays
• Custom validation schemas for each identifier type
• Default validation: email format, phone E.164, username string

Usage

// Email only
const schema = createPasswordIdentitySchema(['email'] as const);

// Multiple identifiers with custom validation
const schema = createPasswordIdentitySchema(['email', 'phone'] as const, {
  phone: z.string().regex(/^\+1[0-9]{10}$/)
});

Related: User story 20250709_user-sign-up.md