[davidluxedesigns]

Summary

Adds two player-stats endpoints the frontend needs: a public online-player count for the home page and per-player rating history for the dashboard.

Closes #1392

Changes

• GET /v1/players/online: public; returns the number of distinct players with an active connection, taken from connection tracking
• GET /v1/players/{id}/rating-history?range=7d|30d|90d|1y|all: rating over time derived from completed games only, ordered oldest to newest; defaults to 30d
• Invalid range returns 400, unknown player returns 404
• Response size is capped to keep the endpoint bounded
• [Index migration / API docs updated, delete whichever doesn't apply]

Testing

• Added tests for online count (zero, multi-tab dedupe, no auth needed)
• Added tests for rating history (completed games only, ordering, range filtering, default range, 400, 404, cap)
• cargo fmt, cargo clippy -- -D warnings and cargo test pass locally

Notes

• Frontend wiring (frontend/app/page.tsx, frontend/app/dashboard/page.tsx) is out of scope for this PR and can follow separately.