Bump finite-identity to production mailer revision

[AustinKelsay]

Summary

• Pin finite-identity to merged main revision 0750e8c, which includes Identity Authority v1 plus production mailer adapter support.
• Refresh Cargo.lock for the new finite-identity dependency metadata.

Verification

• cargo check -p finite-brain-cli
• cargo test -p finite-brain-cli auth_redeem_binds_finite_vip_email_through_identity_authority
• cargo test -p finite-brain-cli auth_redeem_external_email_reports_email_only_brain_limitation
• cargo test --locked
• cargo clippy --all-targets -- -D warnings
• git diff --check