feat(team): use gravatar for avatar when email is set

[gmermoud]

Summary

• Avatar resolution order: explicit picture: field → Gravatar (derived from email: via MD5) → coloured initials
• Falls back to the mystery-person placeholder (d=mp) if no Gravatar is registered for the email
• No new dependencies — uses stdlib hashlib

Test plan

• Verify team members with an email but no picture: field show a Gravatar (or placeholder) instead of initials
• Verify team members with a picture: field still use the explicit photo
• Verify team members with neither email nor picture still show coloured initials