Replace vendored dkim with dkimpy package

[lavr]

emails/packages/dkim/ contains a vendored fork of dkimpy. This creates several issues:

• Performance: the private key is re-parsed on every sign() call, which is suboptimal for bulk sending (see TODO in signers.py:45)
• Maintenance: the fork doesn't receive upstream updates or security fixes
• Type safety: the entire emails/packages/dkim/ directory is excluded from mypy checking

Consider replacing the vendored copy with a dependency on the dkimpy package (or making it an optional dependency like pip install emails[dkim]).