Make email optional on Leadership records

[calebyhan]

Previous leadership records often don't have a known or current email address, but the leadership table's email column was required end-to-end (DB column, create schema, and admin form), which blocked uploading that historical data.

Changes:

• Made email nullable on the Leadership model (was nullable=False)
• Updated LeadershipDTO/CreateLeadershipDTO to allow None for email (UpdateLeadershipDTO already allowed it)
• Removed the required attribute and asterisk from the email field in the admin Leadership form, and send null instead of an empty string when left blank
• Updated Leadership/CreateLeadership/AdminLeadership frontend types to allow null email
• Guarded the mailto: link on the public leadership page so it only renders when an email is present
• Added a test covering leadership creation without an email

Note: existing deployed databases will need a manual ALTER TABLE leadership ALTER COLUMN email DROP NOT NULL since there's no migration tooling in this repo — the schema is created via metadata.create_all().

Closes #207

[github-actions]

Test Results

589 tests  +1   589 ✅ +1   50s ⏱️ +16s
  1 suites ±0     0 💤 ±0 
  1 files   ±0     0 ❌ ±0 

Results for commit d71a960. ± Comparison against base commit b5ceec3.

[MasonMines2006]

LGTM