fix: removed the email check in the signup page

Title fix(signup): allow duplicate/optional email for user registration

Description This merge updates signup email handling for users:

• Removed unique constraint at model level for user_email in user.py.
• Updated signup schema to normalize empty/blank user_email to None so signup works even when email is not provided in user.py.
• Added Alembic migration to drop unique index on users.user_email and recreate it as non-unique in b9c2f8b5c3a1_allow_duplicate_user_email.py.

Impact

• Signup accepts users without email.
• Signup no longer fails if another user already has the same email.

Confirmed: latest commit 36403c2 contains only these 3 files.
Not committed: .env.example, auth_routes.py, config.py.

closes #41 (closed)