feat: add TVL/balance trend sparkline to pool cards

[Yerimahjr]

Closes #142

What this does

• Adds a pool-daily-metrics snapshot writer (/api/cron/snapshot-pool-metrics), triggered daily via Vercel Cron (see vercel.json)
• Adds a batched read endpoint (/api/pools/history) so the Explore/My Groups grid fetches history for all visible pools in one request instead of one per card
• Adds PoolSparkline, a lightweight inline SVG (no new charting dependency), showing the last 30 days of balance history
• Wires it into pool-card.tsx next to the "Total Saved" value

Why no new table/migration

pool_daily_metrics already existed (added in 20260616080000_analytics_schema.sql) and was already being read in /api/analytics, but nothing was writing to it — this PR fills that gap rather than introducing a duplicate table.

Degradation behavior

PoolSparkline returns null for pools with fewer than 2 data points, so new/empty pools just show no sparkline rather than a broken or misleading chart.

Verification

NEXT_PUBLIC_SUPABASE_URL isn't available in my local env, so a full next build isn't possible here — verified instead via tsc --noEmit (no new errors introduced; 14 pre-existing errors in unrelated files remain unchanged) and eslint (clean) on all changed/added files.

Setup needed after merge

A CRON_SECRET env var needs to be added in Vercel project settings for the cron auth check to pass in production.

[Sendi0011]

Great Implementation @Yerimahjr