Optimize community leaderboard ranking

[JoaquinBN]

Summary

• move effective community ranking to database annotations instead of rebuilding all scores in Python per request
• add a profile-specific community context response for top entry, user rank, and nearby rows
• fix community profile rank cache isolation by address
• include Community Link X/Discord points in community scoring while excluding link-only activity from community member metrics
• improve leaderboard pagination with first/previous/next/last controls and real page counts

Verification

• backend/env/bin/python backend/manage.py check
• npm run build

Notes

• npm run check still reports existing repo-wide typing/a11y issues unrelated to this change.
• package-lock.json has an unrelated local workspace name change and was intentionally left out of this PR.