perf: the contest leaderboard response is uncacheable end to end #389

Surfaced reviewing #387, which points the landing hero at GET /api/v1/leaderboard?period=contest. That makes the highest-traffic anonymous surface in the product issue an uncached rebuild against the 512MB free-tier instance that also serves /app and the protocol surfaces.

Nothing on the path caches the response:

• api/routers/leaderboard.py:30 sets no Cache-Control and no ETag.
• dashboard/frontend/vercel.json forces Cache-Control: no-store on /(api|paper|backtest|runs|config|admin|ticker|health|compare)(/.*)?, so the Vercel edge cannot absorb any of it either.
• The caches that do exist in domain/leaderboard/service.py (_cached_run_index, the skip cache) select which run to draw; they do not memoise the built response.

Per request that means all 12 configured entries get a _find_cached_run lookup plus a db.get_equity_curve, then align_equity_curves over the lot. Measured against a local backend on the committed seed DB: 12 entries x 162 points.

The contest window is fixed and historical, so this is the single most cacheable thing the backend produces. A short public, max-age=... on period=contest plus dropping no-store for that one path would remove the load.

Two things to keep in mind for whoever picks this up:

• Do not blanket-cache the route. period=daily reaches maybe_schedule_daily_leaderboard_refresh(), and period=live is the Season 0 preview. Only contest is safely cacheable.
• Related but distinct: perf: unblock event loop in paper-trading / ticker / algo-chat / leaderboard routes #202 covers unblocking the event loop in this route. Caching reduces how often the work runs; it does not make the work non-blocking.