Summary

Adds twitter_handle field to user profiles with case-insensitive uniqueness enforcement and format validation, following the same pattern as github_login.

Closes #102

Changes

• Add migration 005_add_twitter_handle.sql with column and unique index on LOWER(twitter_handle)
• Add twitter_handle field to Profile entity, UpdateProfileRequest, and ProfileResponse DTOs
• Add find_by_twitter_handle repository method with case-insensitive lookup
• Validate Twitter handle format (1-15 alphanumeric characters + underscores)
• Return 400 for invalid format, 409 for duplicate handle
• Include twitter_handle in all GET profile responses
• Update SQLx query cache files

API Changes

PUT /profile

{
  "twitter_handle": "tushar"  // optional, 1-15 chars: a-zA-Z0-9_
}