Add average days early/late project card

[redreceipt]

Summary

• add project schedule variance helpers and aggregate average days early/late for completed projects led by the selected person
• render a new Avg Days Early/Late card in the person-page projects row
• cover the new person-context metric with focused regression tests

Why

Person pages already showed project counts, but they did not show whether completed projects tended to finish early or late. This adds that missing signal directly to the projects row.

Proof

• source .codex-venv313/bin/activate && gunicorn app:app --bind 127.0.0.1:8001
• curl -I -s 'http://127.0.0.1:8001/team/michael?days=7' -> HTTP/1.1 200 OK
• curl -I -s 'http://127.0.0.1:8001/partials/team/michael/content?days=7' -> HTTP/1.1 200 OK
• curl -s 'http://127.0.0.1:8001/partials/team/michael/content?days=7' | rg -n 'Current Projects|Completed Projects|Incomplete Projects|Avg Days Early/Late|on time|[0-9]+d (late|early)|n/a' -> confirmed the new card label and rendered value, including Avg Days Early/Late and 1d late

[linear]

SYM-141